In a bowl, mix the sugar with warm milk until dissolved.

Add the yeast into the mixture, mixing slowly until the yeast is fully dissolved. Let the yeast rest for five minutes until it starts to turn foamy.

In a standing mixer on medium speed, cream the butter, sour cream, sugar, yolks, and vanilla sugar. Add the activated yeast and gradually add the flour. Mix until the dough is soft.

Cover the bowl with Saran wrap and put it in a dark, dry place (like a closed oven). Let the dough rise for two hours until about double in size.

Meanwhile, for the filling, mix the sugars and cocoa and set aside. Separately, blend the oil and sour cream and set aside.

Add all streusel ingredients to a Ziploc bag. Seal the bag and knead until combined.

Once the dough is ready, divide it into three equal parts. Roll out each dough to quarter-inch-thick large rectangles.

Spread one cup of sour cream mixture and sprinkle 1 and 3/4 cups of cocoa mixture on each dough. Roll up the dough lengthwise, jelly-roll-style. Cut the roll in half and twist the two halves together tightly.

Spray three loaf pans with oil spray and place the babkas inside.

Brush the loaves with whisked egg yolks and top generously with streusel.

Bake for one hour and 20 minutes. Let cool completely before removing from the pan. Drizzle with melted chocolate and top with chocolate chunks.