Dough: In the bowl of a stand mixer fitted with a dough hook (or a large mixing bowl if kneading by hand), combine the bread flour, granulated sugar, and instant yeast. Add the warmed whole milk, beaten eggs, orange zest, and vanilla extract, mixing until the dough starts to come together. With the mixer on low speed, gradually add the unsalted butter and fine sea salt a little at a time, mixing well after each addition to fully incorporate. Continue mixing for several minutes, scraping down the sides as needed, and add a small amount of extra flour if the dough is too sticky. The dough should pull away from the bowl and become smooth, though still soft and slightly sticky — refrigeration will make it easier to handle.

First Rise: Transfer the dough to a greased bowl, cover tightly with plastic wrap, and let it rise at room temperature for 1 hour. Then move the dough to the refrigerator to chill for at least 2 hours or up to 24 hours (overnight is ideal).

Chocolate Filling: Melt the bittersweet or semisweet chocolate and butter together in the microwave at 50% power until smooth. Sift in the cocoa powder and powdered sugar, stirring until fully combined. This chocolate filling can be prepared days ahead if desired.

Prep Loaf Pan: Grease an 8.5×4.5×2.5-inch loaf pan with butter and line the bottom with parchment paper.

Roll Out Dough: Remove the dough from the refrigerator and lightly flour your work surface and rolling pin. Roll the dough into a 14×18-inch rectangle, with the shorter side facing you. Work quickly, as the dough is easier to handle when chilled. Spread the chocolate filling evenly over the dough; if needed, gently warm the filling to make it more spreadable.

Shape: Starting from the short edge, roll the dough tightly into a log, sealing the seam at the end. Place the log seam-side down on a baking sheet and refrigerate for 5-10 minutes to firm up for easier cutting. Grease a sharp serrated knife and cut the log lengthwise into two long strips. Position the strips open-side up, crossing one end over the other to form an “X.” Twist the two strips around each other, keeping the chocolate layers facing up, creating a twisted rope. Transfer the twisted loaf into the prepared loaf pan.

Second Rise: Boil water in a small pot and place it on the bottom rack of your oven. Put the babka on the middle rack (with the oven turned off) to rise in the humid environment for 1 hour.

Bake: Remove the babka and water pot carefully from the oven. Preheat the oven to 325°F. Bake the babka uncovered for 30 minutes, then tent it loosely with foil and bake for an additional 30-35 minutes, or until the internal temperature reaches 200°F.

Simple Syrup: While baking, combine water and granulated sugar in a skillet and heat over medium-low until the sugar dissolves completely.

Brush Syrup Over Hot Loaf: Using a pastry brush, generously baste the syrup all over the warm babka’s surface. Use all the syrup. Allow the babka to cool in the pan for 20-30 minutes, then transfer to a wire rack to cool completely.

Slice: Cut the babka with a sharp serrated knife, wiping the blade clean between slices for neat cuts. Babka tastes best the day it is baked but can be stored wrapped in aluminum foil at room temperature for later enjoyment.