Prepare the Brownie Ripple Batter: Preheat oven to 325°F. Lightly coat a 9- x 5- x 3-inch loaf pan with cooking spray. Line bottom and 2 long sides of pan with parchment paper, allowing 2 to 3 inches to extend over sides. Combine butter and chocolate in a microwave-safe bowl. Microwave on HIGH until melted, about 1 minute. Stir until smooth. Whisk in sugar, flour, egg, and vanilla to form a loose batter. Set aside to cool slightly while making cake batter.

Prepare the Pound Cake Batter: Sift together flour, baking powder, and salt into a medium bowl. Set aside.

Beat sugar and butter in a large bowl with an electric mixer on medium-high speed until light and fluffy, 4 to 5 minutes. Beat in vanilla bean paste. Add eggs, 1 at a time, beating well after each addition. Beat in sour cream just until smooth. Add flour mixture, one-third at a time, and beat on low speed just until blended after each addition. Spoon half of the Pound Cake Batter into prepared pan. Spoon Brownie Ripple Batter over Pound Cake Batter, covering it completely. Top with remaining Pound Cake Batter. Using a small offset spatula, smooth batter carefully.

Bake cake in middle of preheated oven until a long wooden pick or a cake tester inserted in center of cake comes out clean, about 1 hour and 45 minutes. (If cake seems to be browning too quickly, cover top loosely with foil for last 30 minutes of baking.) Cool cake in pan on a wire rack 30 minutes. Remove cake from pan using parchment paper as handles, and cool completely on wire rack, about 1 hour. Slice and serve.

Be sure to let the cake cool completely before slicing so the brownie ripple layer has time to set.