Preheat oven to 350°. Place 15 muffin cup liners in muffin tin.
Whisk together first 5 ingredients. Sift mixture through sieve into a bowl; discard any lumps.
Add chocolate, butter, and cocoa to a large heatproof bowl, and pour 1/2 cup boiling water on top. Let sit 2 minutes, and whisk until smooth. Add sour cream, buttermilk, and eggs, whisking until well-combined. Add dry ingredients to wet.
Divide batter among liners; bake in middle of oven 20 minutes or until wooden pick inserted in center comes out clean. Cool 5 minutes; transfer cupcakes to a wire rack to cool. Frost cakes.