Melt the butter with the sugar in a medium saucepan over medium-low heat, stirring occasionally, until the butter is completely melted. Remove from heat, and transfer the mixture to the bowl of an electric mixer fitted with the paddle attachment. Beat on medium-low speed until the mixture is cooled, about 4 to 5 minutes. Add eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed. Blend in the vanilla and then the cocoa mixture until smooth. Reduce speed to low. Add flour mixture in three additions alternating with sour cream, beginning and ending with the dry ingredients and mixing each addition just until incorporated.
Divide the batter evenly between the prepared liners, filling each about ¾-full. Bake until a toothpick inserted in the center comes out clean, about 18 minutes, rotating the pans halfway through baking. Let cool in the pan about 5 to 10 minutes, then transfer to a wire rack to cool completely.
*Nutritional information is an estimate. Please consult a doctor or nutritionist for the most accurate nutritional info.