Whisk together milk, hot water, vegetable oil and sugar in a large bowl. Liquid should be warm but not hot or it will kill the yeast. Whisk in yeast and let sit until yeast foams on top.
Whisk in egg yolks and salt.
Stir in flour a half cup at a time until you reach 3 cups. Then stir in a couple of tablespoons of flour at a time until dough comes together into a cohesive, but still sticky ball. I like to use my hands to I can feel the dough texture better.
Work the dough with your hand for a minute or two to develop the gluten.
Remove the dough from the bowl, lightly coat the bowl in vegetable oil, then return the dough and turn to coat. Cover the dough and let rise at room temperature until doubled in size 1-2 hours.
Lightly coat your work surface in vegetable oil to prevent the dough from sticking, then turn the dough out onto the counter. Using your hands or a well greased rolling pin, gently stretch the dough to a rectangle that is about 18x20 inches in area.
Dot the dough with half of the butter, then use fingers to spread over the entire surface.
Starting at one end of the dough, tightly roll the dough into a log.
Spread half the remaining butter over one half of the log and then fold the log in half, encasing the butter.
Transfer dough to a lined baking sheet and chill in the fridge for 30-40 minutes.
Remove dough from fridge and transfer back to counter. Pat dough out to a thick flat rectangle and then cover half the dough with the remaining butter. Fold the dough in half once more to encase the butter.
Using hands or a well greased rolling pin, stretch the dough to a large rectangle, going as thin as possible without tearing the dough.
Cut the dough in half horizontally and then cut each half vertically into 7 strips, for a total of 14 strips.
Working with one strip at a time, place a strip of 3 squares of chocolate (about 24 grams) at one end of the strip. then roll the dough vertically into a log, encasing the chocolate in the middle. Transfer the rolled buns to baking sheets. 7 buns, per sheet.
Place baking sheets in the fridge and child for 1 1/2 hours.
Preheat oven to 425 F/218 c
Before baking, brush each bun with the reserved egg white and sprinkle liberally with raw sugar.
Bake for 12-14 minutes, rotating the trays halfway through, until buns are golden.
Let cool in pans for 10 minutes, then serve or transfer to racks to cool completely.