Prepare the Dough: In a large mixing bowl, combine the flour, sugar, yeast, and salt. In a separate bowl, mix the warmed milk, melted butter, eggs, and vanilla extract until well combined. Gradually add the wet ingredients to the dry mixture, stirring until a dough forms.
Knead the Dough: Turn the dough onto a floured surface and knead for about 8-10 minutes until smooth and elastic. Form the dough into a ball and place it in a greased bowl, covering it with a clean kitchen towel. Let it rise in a warm area for about 1-2 hours or until it has doubled in size.
Prepare the Filling: In a bowl, combine the chocolate hazelnut spread and cocoa powder. Mix until smooth. If desired, you can also blend in half of the toasted hazelnuts for added texture.
Assemble the Babka: Once the dough has risen, punch it down and roll it out on a floured surface into a large rectangle, approximately 16x24 inches. Spread the chocolate hazelnut filling evenly over the dough. Sprinkle the remaining chopped hazelnuts on top.
Shape the Babka: Starting from one long side, carefully roll the dough into a log. Pinch the seam to seal. Once rolled, carefully slice the log in half lengthwise, exposing the chocolate filling. Twist the two halves together, then coil the twisted dough into a loaf shape and place in a greased 9x5 inch loaf pan.
Second Rise: Cover the pan with a towel and let the babka rise for another 30-45 minutes until puffed.
Preheat Oven: Preheat your oven to 350°F (175°C) during the last 10 minutes of the rising time.
Egg Wash: Beat the egg for the egg wash and brush it over the top of the risen babka for a glossy finish.
Bake: Bake in the preheated oven for 30-35 minutes, or until the babka is deeply golden and cooked through. If the top browns too quickly, cover it with aluminum foil halfway through baking.
Cool and Serve: Remove from the oven and let cool in the pan for about 10 minutes before transferring to a wire rack. Slice once cooled slightly and serve warm or at room temperature.
Notes
Let the babka cool slightly before slicing for the best texture.