Lemon Poppy Seed Cake with Blackberry Filling

This delicious celebration cake features layers of lemon poppy seed cake filled with blackberry filling and iced with lemon Swiss meringue buttercream. You can leave the buttercream as is or colour it and decorate the cake to your liking! The lemon syrup helps to keep the cake moist and adds to the overall lemon flavour. For flat cake layers that you don't need to trim, I recommend using cake strips around the pans while baking. Recipe adapted fromĀ Cook's Illustrated.
Servings: 12 yield(s)
Prep Time: 180 mins
Cook Time: 25 mins
Total Time: 205 mins
Ingredients
    For the Lemon Poppy seed Cake
  • 280 g cake flour (2 1/2 cups)
  • 1 1/4 tsp baking powder
  • 1/4 tsp baking soda
  • 3/4 tsp salt
  • 350 g granulated sugar (1 3/4 cups, divided)
  • 142 g unsalted butter, melted and cooled (10 tbsp)
  • 160 ml whole milk (2/3 cup)
  • 80 ml lemon juice (1/3 cup)
  • 3 tbsp vegetable oil
  • 3 large eggs, separated
  • 3 large egg yolks
  • 1 pinch cream of tartar
  • 2 tsp vanilla extract
  • 2 tbsp lemon zest (from 2 large lemons)
  • 1 tbsp poppy seeds
  • For the Blackberry Filling
  • 500 g fresh or frozen blackberries (4 cups)
  • 134 g granulated sugar (2/3 cup)
  • 1 tbsp lemon juice
  • 1 pinch salt
  • 4 tbsp water, divided
  • 16 g cornstarch (2 tbsp)
  • For the lemon syrup
  • 120 ml water (1/2 cup)
  • 75 g granulated sugar (1/4 cup + 2 tbsp)
  • 2 tbsp lemon juice
  • For the Lemon Swiss Meringue Buttercream
  • 6 egg whites
  • 300 g granulated sugar (1 1/2 cups)
  • 396 g unsalted butter, softened and cubed (1 3/4 cups)
  • 3 tbsp lemon juice, divided
  • 1/2 tsp salt
  • 1 1/2 tbsp lemon extract
  • zest of 2 lemons
Instructions
    For the Lemon Poppy Seed Cake
  1. Adjust your oven rack to the lower-middle position and heat the oven to 350F/180C. Grease, line and flour 3 8-inch/20 cm round cake pans. If using cake strips, soak the strips in cold water.
  2. In a large bowl, whisk together the flour, baking powder, baking soda, salt and 300g (1 1/2 cups) sugar.
  3. In another medium bowl, whisk together the melted butter, milk, 6 egg yolks, lemon juice, oil, vanilla and lemon zest. The lemon juice will curdle the milk, but that is fine.
  4. In a large bowl, beat together the egg whites and a pinch of cream of tartar until foamy.
  5. With the beaters on low speed, gradually pour in the remaining 50 g (1/4 cup) sugar. Increase the speed to medium-high and whip the white until you have glossy, stiff peaks, about 2-3 minutes.
  6. Pour the milk mixture into the flour mixture and beat on low speed until the batter is smooth and fully incorporated, about 30 seconds. Stir in poppy seeds.
  7. Using a rubber spatula, stir in one-third of the whites into the batter. Then add the remaining two-thirds and gently fold until no white streaks remain.
  8. Divide the batter evenly between the three prepared pans and smooth the tops with a rubber spatula. Gently tap the pans on the counter to release any air bubbles. If using cake strips, place them around your pans.
  9. Bake until a toothpick inserted in the centre comes out clean, about 20-25 minutes.
  10. Let the cakes cool in the pans on wire racks for 10 minutes. Remove the cakes from the pans, discard the parchment paper and let cool completely before frosting, about 2 hours.
  11. For the Blackberry Filling
  12. Combine blackberries, sugar, lemon juice, a pinch of salt, and 2 tbsp (30 ml) water in a medium saucepan. Cook over medium heat until the blackberries have broken down and the mixture is cohesive, about 5 - 10 minutes. Use a spoon to mash the softened blackberries to release their juices.
  13. Pour the mixture into a fine-mesh metal strainer that is set over a bowl. Stir the mixture to push the blackberry juice and pulp through the strainer. Discard the seeds.
  14. In a small bowl, whisk together cornstarch with 2 tbsp (30 ml) water.
  15. Rinse out the saucepan and return it to the stove. Add strained raspberry mixture and cornstarch mixture to the saucepan. Heat over medium heat, whisking constantly until mixture is thick and jam-like, about 2-5 minutes. Pour the filling into a bowl and cover with plastic wrap. Chill in the fridge until completely cool.
  16. For the Lemon Syrup
  17. in a small saucepan, combine water, sugar and lemon juice. Bring it to a boil and simmer until sugar is fully dissolved. Remove from heat and cool completely.
  18. For the Lemon Swiss Meringue Buttercream
  19. Place egg whites, 2 tbsp lemon juice and sugar into the bowl of a stand mixer, whisk until combined.
  20. Place the bowl over a double boiler on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F/71C on a candy thermometer.
  21. Place the bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch and the temperature of the meringue is 69-72 F/21-22 ). This could take up to 15 minutes. If the mixture is still not cool enough after 15 minutes, chill in the fridge for 5-10 minutes until it is cool enough. It is important that the mixture is not hot, or you will end up with soupy or too soft buttercream.
  22. Slowly add the cubed butter, one piece at a time, and mix until smooth.
  23. Add remaining 1 tbsp lemon juice, the lemon extract, lemon zest and salt and beat to combine. The buttercream may be kept covered at room temperature for up to 12 hours.
  24. For Assembly
  25. Trim cake layers so that the tops are flat, if necessary.
  26. Place one cake layer on a cake board and brush the top with lemon syrup.
  27. Spread a thin layer of buttercream over the top of the cake and then pipe a thicker band to border the sides of the cake. Spread half of the filling inside the buttercream border. Repeat with 2nd layer of cake, lemon syrup, buttercream and filling.
  28. Top with 3rd layer of cake. Soak the cake with lemon syrup then top with a layer of buttercream.
  29. Check the layers on the sides of the cake. To hold the cake layers in place while you do the crumb coat, you can stick a skewer down through the centre of the cake. Coat the sides in a thin layer of buttercream. Chill the cake for 30 minutes until the buttercream is solid before adding your final coat of buttercream and decorating the cake.