Preheat Oven to 325 degrees Fahrenheit
Lightly coat a 9 x 5 x 3 inch loaf pan with cooking spray. Line bottom and 2 long sides with parchment paper, allowing paper to extend over sides.
Prepare the Brownie Ripple Batter: 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. Using a small spatula, smooth batter carefully
Bake cake in middle of preheated oven until a long wooden pick or 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 pano in a wire rack for 30 minutes. Remove cake from pan using parchment paper as handles, and cool completely on wire rack, about 1 hour. Slice and serve.