1. Dissolve yeast in 1 1/2 cups warm water in a small bowl; let stand 5 minutes.
2. Weigh or lightly spoon flours and flaxseed meal into dry measuring cups; level with a knife. Combine flours, flaxseed meal, cocoa, and next 5 ingredients (through salt) in the bowl of a stand mixer fitted with a paddle attachment; beat at medium speed until blended. Add yeast mixture, butter, molasses, vinegar, and eggs; beat until blended. Spoon batter in a 9 x 5-inch loaf pan coated with cooking spray. Cover loosely with plastic wrap, and let rise in a warm place (85°), free from drafts, 1 hour or until dough is just above top of pan.
3. Preheat oven to 400°.
4. Bake at 400° for 20 minutes. Cover top of bread with foil; bake an additional 25 minutes. Remove foil from bread; bake 5 minutes. Cool 10 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.