Preheat the oven to 350℉. Grease a 9x5-inch loaf pan with butter or non-stick spray.

In a bowl using an electric mixer, cream together the softened butter and white sugar for 1 minute. Add the room-temperature eggs and vanilla and continue creaming at high speed until fluffy. Add the sour cream and mix just until incorporated.

In a separate bowl, whisk together the flour and baking powder before adding to the combined wet ingredients and mixing at low speed just until no streaks of flour remain.

Melt ¼ cup of chocolate chips in a heat-safe bowl in 15-second increments, stirring between each increment until smooth and completely melted. Fold half of the batter into the melted chocolate along with the cocoa powder just until smooth.

Alternate scoops of both the chocolate and vanilla into the greased loaf pan using a cookie scoop or teaspoon until all the batter is used up. Use a butter knife to gently swirl the batter around throughout the loaf to create a marbled loaf being careful not to overmix so that each batter still has definition.

Bake in the preheated oven for 45-55 minutes or until a toothpick inserted into the center comes out clean. Remove from the pan after 10 minutes and let cool while making the icing.

Melt the butter and ¼ cup of chocolate chips for the icing in 15-second increments, stirring between each increment until fully melted. Then, whisk in the powdered sugar, cocoa powder, vanilla, and enough milk to make a spreadable yet not runny icing. Spread over the cooled loaf. Let the icing set for about 15 minutes before slicing to serve for clean slices or dig right in! Enjoy!