Preheat oven to 350 degrees F (175 degrees C). Grease a 9-inch baking pan with cooking spray.
Melt chocolate chips in a microwave-safe glass or ceramic bowl in 15-second intervals, stirring after each melting, about 1 minute. Let cool.
Combine sugar and egg in a large bowl; beat with an electric mixer until light and airy, about 3 minutes. Add melted chocolate, flour, milk, cocoa powder, sour cream, and vegetable oil; beat until batter is smooth and well blended.
Pour batter into the prepared baking pan.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 30 minutes.