Chocolate Babka

Whisk together yeast and milk in the bowl of a standing mixer until dissolved. Add 3⁄4 cup flour and beat on slow speed with paddle attachment; increase speed to medium-high and beat for 3 minutes. Cover with plastic wrap and let rise in a very warm spot until doubled in volume, about 1 hour. Add 1 1⁄3 cups flour, 1⁄2 cup sugar, 1⁄2 tbsp. salt, zests, 1⁄2 tsp. vanilla, and yolks to dough. Beat on slow speed to combine; increase speed to medium and mix until dough pulls away from paddle, then increase to high and beat for 2 minutes. Reduce to low and drizzle in 8 tbsp. melted butter in 3 batches, waiting between additions until it's absorbed; beat until combined. Increase to high; beat for 3 minutes more. Cover bowl and let rise in a very warm spot until doubled in volume, about 2 hours. Punch down dough with floured hands; turn out onto a parchment paper–lined sheet pan dusted with flour; dust top of dough with flour. Slip pan into a garbage bag and tie tightly; chill overnight.

Beat almond paste and egg white in a bowl with an electric mixer until combined. Add crumbs, 1⁄2 tbsp. melted butter, and 1⁄4 cup water; beat until combined. Cover and set filling aside. Stir together remaining flour, 1⁄4 cup sugar, and 1⁄2 tsp. cinnamon in a bowl. Add diced butter and remaining vanilla; beat just until crumbly. Cover and chill streusel. Grease two 2 1⁄4" × 4" × 8" loaf pans with a little of the remaining melted butter. Combine raisins and rum in a bowl; then combine remaining sugar and cinnamon in another bowl. Whisk together remaining salt, 1 egg, and 2 tsp. water in a bowl to make an egg wash.

Halve chilled dough. On a floured surface, roll out each dough piece to a 6" × 14" rectangle (long sides parallel to you). Brush with egg wash; divide and spread half of the filling over each, leaving a 1" margin around edges. Spread 1⁄4 cup chocolate over each (reserve remaining chocolate); sprinkle with the raisin mixture and cinnamon–sugar mixture. Press raisins and filling into dough. Roll each dough half into a tight jelly roll; press to seal bottom and sides. Gently roll each jelly roll into a 20"-long piece. Fold each in half lengthwise and twist each twice. Place each twisted roll in a prepared pan, seam side down and ends tucked under. Brush tops and sides with the remaining melted butter; sprinkle with two-thirds of the streusel (reserve remaining streusel), gently pressing it in. Let rise in a very warm spot until doubled in volume, about 2 hours.

Heat oven to 350°. Sprinkle loaves with remaining streusel and bake until golden brown and cooked through, about 45 minutes. Let cool. Drizzle with remaining chocolate and let set.