Dissolve 1 tsp. sugar and yeast in warm soy milk in the bowl of a stand mixer; let stand 5 minutes. Stir in 6 tbsp. sugar, vanilla, 1/4 tsp. salt, and egg yolk. Lightly spoon flours into dry measuring cups: level with a knife.
Add 1 1/3 cups flour to milk mixture; beat with dough hook attachment at medium speed until well blended (about 2 minutes). Add 5 tbsp. margarine, 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 remaining flour mixture as needed, 1 tbsp. at a time, to prevent dough from sticking to hands. Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place, 1 1/2 hours or until doubled in size.
Punch dough down; cover and let dough rest 5 minutes. Line the bottom of a 9 x 5-inch loaf pan with parchment paper; coat sides of pan with cooking spray.
To prepare filling, combine 1/2 cup sugar, cocoa, cinnamon, salt, and chocolate in a medium bowl, set aside.
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 ends to seal.
Holding dough by ends, twist dough 4 times as if wrining out a towel. Fit dough into prepared pan. Cover and let rise 45 minutes or until doubled in size.
Preheat oven to 350F.
To prepare streusel, combine powdered sugar, 1 tbsp. all purpose flour, and 1 tbsp. 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 completely on wire rack before slicing.