2 tbsp cold butter, cut into small cubes
For glazing:
1 egg white, beaten
Method
To make the cake: add the yeast to a stand mixer with the water and 1 teaspoon of sugar. Leave to activate for about 10 minutes.
Add the ⅓ cup sugar, milk, vanilla extract and softened butter.
Add the salt and the 2½ cups of flour gradually while mixing on low speed.
Mix on medium speed for about 5 minutes and you get a flexible and smooth dough.
Add the eggs and then the egg yolks, then another 1½ cups flour gradually and keep mixing until it forms a soft dough.
Flour a work surface with ½-1 cup flour, and knead the dough for a few minutes until it forms a smooth and velvety dough, adding flour as necessary, only enough so that it doesn't stick.
Transfer the dough to a oiled bowl and cover with cling film and let rise for about an hour in a warm place, until doubled in size.
Meanwhile make the cocoa mixture: mix the cocoa powder and sugar together in a small bowl until combined and set aside.
Place the risen dough on a floured surface, and press down with a fist.
Divide the dough into two and roll each half into a 25x50cm rectangle.
Brush the melted butter on each rectangle, leaving a 2cm margin.
Sprinkle the cocoa mixture over the melted butter. Ensure there’s a very generous amount of chocolate to make sure there’s a good amount of chocolate coverage even after the cake expands once it rises and bakes.
Brush a little beaten egg white on the margins to help them stick.
Roll each rectangle along its side into a tight roll, and pinch the ends to seal.
Oil the tins and add each rectangle to the tins, twisting each end left and right multiple times (in an “s” shape, in alternating opposite directions) until it resembles a corkscrew, adding more chocolate as you roll.
Cover and let rise until the dough doubles in size, about 35-45 minutes.
Meanwhile make the streusel: add all the dry ingredients to the bowl of a food processor, then add the butter and then pulse a few times until the mixture forms coarse crumbs.
Preheat the oven to 180°C.
Brush each cake with beaten egg white, and scatter the streusel on top.
Bake for 30-35 minutes until golden brown.
Remove from the oven and let sit 5 minutes in the mould, then remove carefully and let cool on a rack.