Set the oven to 350.
Spray the cupcake tin[s] with baking spray, or grease and lightly flour, or line with cupcake papers.
Sift the flour, cocoa powder, baking powder, and soda into a medium bowl.
Add salt, and set aside.
In the bowl of an electric mixer, whip the butter for a moment.
Add the sugars and cream on medium until light and fluffy.
Gradually add the eggs and then the melted chocolate until nicely blended.
Add one-third of the dry ingredients and then 1/4 c of buttermilk and the vanilla and beat until smooth.
Add another third of the flour mixture and another 1/4 c buttermilk and beat until incorporated.
Add the final third of the flour mixture and the final 1/4 c buttermilk and mix on low until combined.
Incorporate the sour cream or yogurt.
Fill cupcake tins only about one-third full and bake for 18-20 minutes, or until a toothpick comes out clean.
Cool cakes in pan[s] for 10 minutes.
Remove to wire rack and cool completely.
[Clean, re-coat or line tin, re-fill, and repeat as necessary.]