For the batter: Preheat the oven to 350 degrees F. Beat the butter, brown sugar
, sour cream, egg
and vanilla with 1/2 cup water in a stand mixer fitted with the paddle attachment on medium speed until blended.
Sift together the flour, cocoa powder, baking powder, baking soda, cayenne and salt in a medium bowl. Add the dry mixture to the batter and mix on high until blended, 2 minutes.
Place paper liners in a cupcake pan and scoop the batter into the liners. Bake until the tops bounce back when pressed, about 18 minutes. Let the cupcakes cool fully before frosting (about 1 hour).
For the spicy buttercream: Beat together the sugar, butter, cream, cocoa and cayenne with an electric mixer until smooth, about 3 minutes. (It should be stiff, not runny.) Use a piping bag or spoon to spread the buttercream on top of the cooled cupcakes.