Combine first 3 ingredients in a large bowl; beat with a mixer at medium speed until light and fluffy. Add eggs, beating until incorporated. Stir in 1/2 teaspoon vanilla.
Weigh or lightly spoon flours into dry measuring cups; level. Combine flours, powder, soda, and 1/4 teaspoon salt. Combine sour cream and 1/4 cup milk. Add flour mixture and sour cream mixture alternately to butter mixture, beginning and ending with flour mixture.
Combine 1 teaspoon canola oil and chopped bittersweet chocolate in a microwave-safe dish. Microwave at HIGH for 1 minute, stirring every 20 seconds until smooth. Stir in 2 tablespoons cocoa. Remove 2 1/2 cups vanilla batter; add to chocolate mixture, and stir.
Using 2 ice-cream scoops or 2 large spoons, add batters alternately to a 10-cup Bundt pan coated with baking spray; swirl.
Bake at 350° for 50 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 15 minutes. Remove from pan; cool on wire rack.
Combine powdered sugar, 2 tablespoons milk, 1 tablespoon cocoa, 1/4 teaspoon vanilla, and 1/8 teaspoon salt; stir until smooth. Drizzle glaze over cake.