From the blog For Love of the Table
3/4 c. warm whole milk (105° to 115°)
1/2 c. plus 2 t. sugar
1 T. active dry yeast
3 2/3 c. all-purpose flour (415 grams) plus additional for dusting
2 whole large eggs
1 large egg yolk
1 t. vanilla extract
3/4 t. salt
1 1/4 sticks (10 T.) unsalted butter, cut into pieces and softened slightly, but cool
1 large egg yolk
1 T. whole milk
5 T. unsalted butter, divided
2 (3 1/2- to 4-oz) bars fine-quality semi-sweet chocolate, finely chopped and divided
1/4 c. sugar, divided
Stir together warm milk and 2 teaspoons sugar in bowl of mixer. Sprinkle yeast over mixture and whisk to blend. Let stand until foamy, about 5 minutes.
Add 1/2 cup flour to yeast mixture and beat at medium speed until combined. Add whole eggs, yolk, vanilla, salt, and remaining 1/2 cup sugar and beat until combined. Reduce speed to low, then mix in the remaining flour, about 1/2 cup at a time. Increase speed to medium, then beat in butter, a few pieces at a time, and continue to beat until dough is shiny and forms strands from paddle to bowl—4 to 5 minutes. (Dough will be very soft and sticky.) Scrape dough into a lightly buttered bowl and cover bowl with plastic wrap. Let dough rise in a draft-free place at room temperature until doubled in bulk, 1 1/2 to 2 hours. Gently deflate the dough with a buttered or oiled spatula. Cover again with plastic wrap and refrigerate for at least 4 hours or overnight.
Lightly butter two 6 cup loaf pans. Line the pans with 2 pieces of parchment paper (1 lengthwise and 1 crosswise).
Remove the dough from the refrigerator. Cut the dough in half. Return one piece to the refrigerator while you roll out the first loaf. Beat together yolk and milk and set aside.
Melt 2 1/2 T. of the butter. Roll out the first piece of dough on a well-floured surface with a lightly floured rolling pin into an 18- by 10-inch rectangle and arrange with a long side nearest you. Spread the butter over the dough, leaving a 1/2-inch border across the long edge furthest from you. Brush the unbuttered edge with egg wash. Sprinkle half of chocolate evenly over buttered portion of the dough and then sprinkle with half of sugar (2 tablespoons). Starting with long side nearest to you, roll dough into a snug log, pinching firmly along the seam to seal. Bring ends of log together to form a ring, pinching to seal. Twist entire ring twice to form a double figure 8 and fit into one of lined loaf pans. Make another babka with remaining dough, butter, egg wash, chocolate and sugar in same manner. Chill remaining egg wash, covered, to use later. Loosely cover the pans with buttered plastic wrap (buttered side down) and let the babkas rise in a draft-free place at room temperature until the loaves look puffy and an indentation remains when the dough is lightly pressed—1 to 2 hours. Put oven rack in middle position and preheat oven to 350° (if using Pyrex pans, preheat the oven to 325°).
Brush the loaves with the remaining egg wash. If desired, sprinkle the loaves lightly with Turbinado sugar. Bake until tops are deep golden brown and bottoms sound hollow when tapped (when loaves are removed from pans), about 40 to 55 minutes. Transfer loaves to a rack and cool to room temperature. Babkas will keep, wrapped in plastic wrap and then foil, frozen for 3 weeks.
(Recipe from Gourmet, December 2006)
Variations
Streusel Topping: Many babka recipes add a streusel topping. If you would like to top your babkas with streusel combine 3/4 c. confectioner's sugar and 2/3 c. all-purpose flour in a bowl. Rub in 6 T. of butter. Squeeze so that you have some large clumps in addition to small crumbs. This is enough streusel for 2 loaves. Scatter the streusel over the egg-washed loaves (omit the Turbinado sugar) and bake. (Streusel adapted from Martha Stewart)
Filling Variations: Don't feel limited to a chocolate filling. Fillings can include nuts, dried fruits, spices, etc. Be creative. The "Baby Babkas" pictured in this post have chopped white chocolate (4 oz.), grated almond paste (2 oz.) and dried cranberries (1/2 c.) soaked in a small amount of orange juice and drained (quantities are for half a recipe or 8 baby babkas).
Baby Babkas: Instead of 2 loaves, make 16 "baby" babkas. Divide the dough and roll out and fill as described for loaves. Instead of rolling the entire sheet of dough jelly roll-style, cut each 18- by 10-inch rectangle horizontally into two strips; roll up each of these strips jelly roll-style and cut each of these narrow "rolls" into four equal pieces. Twist each piece attractively into a small bun. Alternatively, cut each 18- by 10-inch rectangle in half vertically and then cut each half into four equal strips horizontally. Roll each of these miniature strips up jelly roll style and coil them up like a snail, tucking the end under. Place each "baby babka" into a well buttered 6-oz. ramekin. Let rise as for the large loaves. Apply egg wash and sugar. Baking time will be shorter—25 to 30 minutes total. Cool for 5 minutes before removing from the ramekins.
http://www.forloveofthetable.com/2012/12/chocolate-babka.html