Dissolve the granulated sugar and yeast in warm milk in the bowl of a stand mixer; let stand 5 minutes.
1 package active dry yeast, ¾ cup warm 1% low-fat milk, 1 teaspoon granulated sugar
Prepare the Dough:
Stir in the granulated sugar, vanilla extract, salt, and egg yolk.
6 tablespoons granulated sugar, ½ teaspoon vanilla extract, ¼ teaspoon salt, 1 large egg yolk
Add 6 ounces (about 1 ⅓ cups) all purpose flour and the bread flour to milk mixture; beat with dough hook attachment at medium speed until well blended (about 2 minutes).
Add butter, beating until well blended. Scrape dough out onto a floured surface. The dough will be very sticky. Knead until smooth and elastic (about 10 minutes); add 1.5 ounces (about ⅓ cup) all purpose flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will be very soft). **I used my mixer to knead the dough. I just set it to medium speed and used the dough hook to knead for 10 minutes. I added the 1.5 ounces of flour, 1 tablespoon at a time, to the dough every minute or so, until it was completely incorporated.
5 tablespoons butter
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 ½ hours or until doubled in size. Gently press two fingers into dough. If indentation remains, dough has risen enough. Punch dough down; cover and let dough rest 5 minutes. While the dough is rising, prepare the filling.
Prepare the Filling and Assemble the Loaf:
For the filling, combine the granulated sugar, cocoa, cinnamon, salt, and chocolate in a medium bowl; set aside.
½ cup granulated sugar, 3 tablespoons unsweetened cocoa powder, ½ teaspoon ground cinnamon, ¼ teaspoon salt, 4 ounces semisweet chocolate
Line the bottom of a 9 x 5–inch loaf pan with parchment paper; coat sides of pan with cooking spray.
Place dough on a generously floured surface; roll dough out into a 16-inch square. Sprinkle filling over dough, leaving a ¼-inch border around edges. Roll up dough tightly, jelly-roll fashion; pinch seam and ends to seal. Holding dough by ends, twist dough 4 times as if wringing out a towel. Fit dough into prepared pan.
Cover and let rise 45 minutes or until doubled in size. Meanwhile, preheat oven to 350°F and prepare streusel.
Prepare the Streusel and Bake the Babka:
For the streusel, combine powdered sugar, all purpose flour, and cold butter, stirring with a fork until mixture is crumbly.
Sprinkle streusel evenly over dough. Bake for 40 minutes or until loaf is browned on bottom and sounds hollow when tapped.
Cool bread in pan 10 minutes on a wire rack; remove from pan. Cool bread completely on wire rack before slicing.
Notes
Source: Adapted from Cooking LightNutritional information is only an estimate and will vary based on your ingredients and substitutions. You're Gonna Bake It After Allbakeitafterall.com