Preheat the oven to 350 degrees F. Line two muffin tins with 18 cupcake liners.
Pass the cocoa powder
, all-purpose and cake flours, baking soda
and baking powder through a fine-mesh sieve
into a large bowl. Whisk in the granulated and light brown sugars, orange zest
and salt until thoroughly combined. In a medium bowl, whisk together the eggs, sour cream, water, melted butter and vanilla until smooth. Add the flour mixture to the egg mixture and whisk
Divide the batter
among the cupcake liners, filling each by about two-thirds. Bake until set and a toothpick comes out clean, about 20 minutes. Transfer to a wire rack to cool.
Frost the cupcakes with the buttercream and garnish
with candied orange peel.