Heat oven to 350 degrees. Line standard-sized muffin pan with baking-cup liners.
Combine butter, chocolate, and cocoa in medium heatproof bowl. Set bowl over saucepan containing barely simmering water; heat mixture until butter and chocolate are melted and whisk until smooth and combined. Set aside to cool until just warm to the touch.
Whisk flour, baking soda, cinnamon, and baking powder in small bowl to combine.
Whisk eggs in second medium bowl to combine; add sugar, vanilla, and salt and whisk until fully incorporated. Add cooled chocolate mixture and whisk until combined. Sift about one-third of flour mixture over chocolate mixture and whisk until combined; whisk in sour cream until combined, then sift remaining flour mixture over and whisk until batter is thick.
Divide batter evenly among muffin pan cups. Bake until skewer inserted into center of cupcakes comes out clean, 18 to 20 minutes.
When the cupcakes have cooled, mix together a half teaspoon of cinnamon with 3 tablespoons powdered sugar. Sift a light coating of the mixture on top of the cupcakes.