Activate the Yeast: In a small bowl, combine the active dry yeast with lukewarm milk (105-115°F) and stir gently. Let sit for about 10 minutes until frothy.

Make the Dough: In a large mixing bowl, whisk together the flour, sugar, and salt. Add the yeast mixture, egg, butter, and vanilla. Mix until a dough forms, then knead on a floured surface for 8-10 minutes, adding chocolate chunks.

First Rise: Place the dough in a greased bowl, cover, and let rise in a warm area for 1-2 hours, until doubled.

Shape the Buns: Punch down the dough, divide into 8 pieces, and shape into ovals. Let them puff up for 30 minutes on a lined baking sheet.

Preheat Oven: Preheat the oven to 350°F (180°C).

Prepare Egg Wash: Beat together the egg yolk and milk. Brush over the buns.

Decorate the Buns: Cut slits into the tops of each bun to create a decorative pattern.

Bake: Place buns in the oven and bake for 15-20 minutes until golden brown and hollow sounding when tapped.

Cool and Serve: Let the buns cool on a wire rack for a few minutes and serve warm.