Place the yeast and water in a bowl for 10 minutes, for yeast to activate.

In the meantime, beat the butter and sugar in a standing mixer with a dough hook, at medium speed until softened, about 5-7 minutes. Scrape down the sides as necessary.

Add the following one a time, allowing each addition to combine properly: egg yolks, buttermilk, vanilla, salt, and the yeast mixture.

Add the flour to the mixer in 3 additions, mix until the dough is smooth and no longer sticking to the sides of the mixer, about 5-10 minutes.

Transfer the dough to a lightly greased bowl and cover with a humid cloth. Place somewhere warm and allow to rise for 1 1/2 hours, until the dough has doubled in size.

Prepare a sheet pan covered with parchment paper. Prepare egg wash by mixing the egg yolk with a teaspoon milk.

After the dough has doubled in size, punch the dough down to remove the excess air. Using a rolling pin, roll the dough out to a rectangle shape.

Brush the brioche dough with melted butter, sprinkle the sugar on top followed by the chocolate chips.

Brush egg wash on the long end of the rectangle nearest to you and roll the dough inward, tightly, into a log. Slice the roll vertically down the middle. Twist the two long strips and shape into a ring. Gently place on the sheet pan and allow to rise for 1 hour.

Heat the oven to 350F. Brush the top and sides of the babka with the remaining egg wash.

Bake in the oven for 30-40 minutes until the top is golden.