Make the dough: Combine the yeast and 2 tablespoons warm water in the bowl of a stand mixer; set aside until foamy, 5 minutes. Add the milk, sour cream, egg yolk and 1/2 cup flour; beat with the paddle attachment on medium speed until combined. Gradually beat in the remaining 1 1/4 cups flour, scraping the bowl as needed, to form a wet dough, about4 minutes. Increase the speed to medium high; add the whole egg, sugar, salt and vanilla and beat until creamy, 3 to 4 minutes. Add the butter, 1 tablespoon at a time; beat until incorporated. The dough will be very wet.

Brush a large bowl with butter; transfer the dough to the bowl and cover with plastic wrap. Let rise in a warm place until nearly doubled in size, about 1 hour, 30 minutes. Stir the dough to punch it down; cover and refrigerate until firm, at least 3 hours or up to overnight.

Generously dust a sheet of parchment paper with flour. Pat the dough into a square on the parchment, then roll into an 11-by-15-inch rectangle. Transfer the dough and parchment to a baking sheet and brush off the excess flour. Cover and chill until firm enough to shape, 1 to 2 hours.

Meanwhile, make the filling: Pulse the almonds and sugar in a food processor until fine. Add the butter, egg, rum, vanilla, cinnamon, orange zest, cocoa powder and almond extract; pulse until smooth. Cover and chill 1 hour.

Make the streusel: Mash the sugar, flour and butter in a bowl with a fork to make clumps. Cover and chill until ready to use.

Butter an 8 1/2-by-4 1/2-inch loaf pan.  Bring the chilled dough out of the fridge. Spread the coco/almond filling over the dough, leaving a 1-inch border; sprinkle with the chopped chocolate. Starting from a short side, use the parchment to lift and tightly roll up the dough, brushing off the excess flour. Pinch the ends of the roll, then twist the entire roll of dough several times (like wringing out a towel, sort of). Fold the twisted roll in half so the ends meet. Twist the folded dough a couple of turns and place in the loaf pan.  Cover with buttered parchment; let rise in a warm place until puffy, about 2 hours. (Note: To make my babka more dense, like the Zadies Break-away, I omitted this final rising time and just went straight to adding the streusel and baking.)

Preheat the oven to 350 degrees F. Brush the loaf with the beaten egg, then sprinkle with the streusel. Bake until the bread is golden and springs back when pressed, 1 hour to 1 hour, 10 minutes. Loosen with a knife, then let cool in the pan, 1 hour. Unmold onto a rack to finish cooling.