Heat your milk to just lukewarm and add one or two teaspoons of the sugar and the yeast. Stir, and let proof until foamy.
Combine 228 grams or 1 3/4 cups flour, butter, remaining sugar, the egg, and vanilla into the bowl of your stand mixer, or a large mixing bowl. Add in foamy yeast and milk mixture.
Place on your stand mixer, fitted with a dough hook, and turn on low. You can also mix by hand.
When a dough has formed and the flour is absorbed, turn the mixer to high, or begin to knead and work the dough. Add flour in tablespoons as needed if dough is too sticky to handle, or sticking to the sides and bottom of the bowl.
Knead until soft, tacky, and smooth, 7 to 8 minutes on the stand mixer, and around 10 by hand.
Cover and let rise in a warm place for about an hour, or until doubled in size.
Make the Filling
In the last 20 minutes of your dough's first rise, combine all the filling ingredients in the bowl of your stand mixer.
Blend until it forms a sticky, grainy paste, similar to wet sand. Set aside.
Shaping and Baking
When the dough is risen, turn out onto a clean non stick surface, or lightly flour work surface. Gently shape the dough into a 15 x 12 inch rectangle.
Working from top to bottom along the 15 inch side, cut one inch strips from the edge of the dough to about 1/3 of the way in.
Repeat on the other side, leaving the middle third of the dough for the chocolate filling.
Place the chocolate filling along the middle of the dough, leaving 1/2 inch at the top and bottom. Mold the filling evenly and flatten it gently to about 1 inch thick, before braiding for best results and even chocolate distribution.
Begin folding, starting by tucking the top 1/2 inch down over the top edge of the chocolate filling.
Working from one side to the other gently lay strips across the filling, pressing the end into the base of the strip diagonally below it.
Repeat until you reach the final strip and the bottom edge. Fold the bottom edge up over the chocolate filling before the final strip of dough.
Stretch the final strip of dough across the dough, securing it gently underneath the loaf. Transfer to a parchment or silicone lined baking sheet.
Let rise again, covered by a towel or gently oiled plastic wrap, until very puffy, about 45 minutes to 1 hour. Preheat your oven to 350°F after 35 minutes.
Gently but generously brush milk on the loaf, then bake for 20 to 25 minutes, or until the top is golden and crisp and the bottom sounds hollow to when tapped.
Transfer to a cooling rack right away and allow to cool before decorating.
When ready, melt your chocolate of choice in the microwave, stirring every 20 seconds until it's smooth and glossy.
Drizzle over the loaf as desired. Serve, or let the chocolate harden, and store in an airtight container, or tightly wrapped, for up to 3 days.
Notes
*You'll need a bit of extra milk for brushing on the loaf before baking as well.