Mix milk, sugar and yeast in a bowl and let rest for about 10 minutes.
When the yeast is bubbly, add the remaining ingredients and knead to a dough. If the dough is too sticky, add up to another 50g of flour. The dough should be very soft and elastic.
Place the dough in a greased bowl and let rise, covered, for about 2 hours.
Alternatively, the dough can rest overnight in the refrigerator.
for the brownie
Melt the butter and chocolate in a small saucepan over low heat.
Beat the eggs, sugar and vanilla in a bowl until fluffy.
Add the chocolate to the egg mixture. Add the rest of the ingredients and mix.
for the babka
Preheat the oven to 190°C.
Grease the loaf pan with a little oil.
Roll out the yeast dough into a rectangle and spread the brownie batter on it.
Roll up the dough from the short side.
Cut the roll in half lengthwise and twist both parts together.
Place the dough in the loaf pan and let it rise in a warm place for about 45 minutes.
Bake the babka for 30-40 minutes. If the surface gets too dark, cover with a piece of aluminum foil.
Notes
You can find the original recipe and more notes at Buttermilk by Sam, be sure to check her out.