Dough: Add flour, sugar, and yeast to the bowl of a stand mixer with a dough hook (or a large mixing bowl if making by hand). Add milk, eggs, zest, and vanilla and mix well until the dough comes together. With the mixer on low, add the butter and salt, just a little at a time, mixing really well after each addition to fully incorporate.

Knead: Continue mixing the dough for several minutes, scraping the sides of the bowl as needed (and add a little extra flour if needed), until the dough pulls away from the sides of the bowl and is smooth. It will be very soft and still slightly sticky (refrigeration will help make it easier to handle).

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

Chocolate Filling: Melt chocolate and butter in the microwave at 50% power until melted and smooth. Measure the cocoa powder and powdered sugar and then sift into the melted chocolate and stir to combine. The chocolate filling can be made days in advance (see notes).

Prep Loaf Pan: Grease an 8.5x4.5x2.5 inch loaf pan with butter and place a piece of parchment paper at the bottom.

Roll Out Dough: Remove dough from the fridge. Lightly flour your counter and rolling pin and roll dough to about 14x18inch rectangle (short side parallel to you). Work fast, as it's easier to handle when it's chilled.

Spread chocolate all over the top of the dough (rewarm the chocolate slightly in the microwave, if needed, to make it spreadable).

Shape: Starting at the short end, roll dough tightly into a log, sealing the long seam at the end and place seam-side-down on a baking sheet. Refrigerate log for 5-10 minutes, to make it easier to cut.

Grease a sharp serrated knife and cut the dough lengthwise, into two long strips. Place strips open-side-up and cross one end over the other, to make an “x”, then twist both ends around each other, to form a twisted rope (chocolate layers facing up). Place into prepared loaf pan.

Second Rise: Boil water in a small pot, then place pot of water into the bottom of your oven. Place babka on middle rack in the oven (oven is off) and allow to rise for 1 hour (the hot water will create a humid environment in the oven while the loaf rises).

Bake: Carefully remove risen dough and water from oven. Preheat oven to 325 degrees F. Once preheated, bake babka at 325 degrees F for 30 minutes uncovered, then tent with a piece of foil and bake for about 30-35 more minutes, or until bread reaches 200 degrees F.

Simple Syrup: While bread is baking, make simple syrup by mixing water and sugar in a skillet. Cook over medium-low until sugar is dissolved.

Paint Syrup Over Hot Loaf: Use a pastry brush to gently baste syrup all over the top of the loaf. Use all of the syrup! Allow the babka to cool in the pan for 20-30 minutes then remove to a wire cooling rack to cool completely.

Slice: Use a sharp serrated knife to cut the loaf, wiping knife clean between each slice.

Babka is best enjoyed the day it’s baked, but store leftovers wrapped in aluminum foil, at room temperature.