1. Dissolve 1 teaspoon granulated sugar and yeast in warm milk in the bowl of a stand mixer; let stand 5 minutes. Stir in 6 tablespoons granulated sugar, vanilla extract, 1/4 teaspoon salt, and egg yolk. Weigh or lightly spoon flours into dry measuring cups; level with a knife. Add 6 ounces (about 1 1/3 cups) all-purpose flour and bread flour to milk mixture; beat with dough hook attachment at medium speed until well blended (about 2 minutes). Add 5 tablespoons butter, beating until well blended. Scrape dough out onto a floured surface (dough will be very sticky). Knead until smooth and elastic (about 10 minutes); add 1.5 ounces (about 1/3 cup) all-purpose flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will be very soft).
2. 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 1/2 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.
3. Line the bottom of a 9 x 5–inch loaf pan with parchment paper; coat sides of pan with cooking spray.
4. To prepare filling, combine 1/2 cup granulated sugar, cocoa, cinnamon, salt, and chocolate in a medium bowl; set aside.
5. Place dough on a generously floured surface; roll dough out into a 16-inch square. Sprinkle filling over dough, leaving a 1/4-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.
6. Preheat oven to 350°.
7. To prepare streusel, combine powdered sugar, 1 tablespoon all-purpose flour, and 1 tablespoon softened butter, stirring with a fork until mixture is crumbly; sprinkle streusel evenly over dough. Bake at 350° 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.