Preheat oven to 350 degrees. Line cupcake tin with baking cups, or grease with cooking spray.
Melt chocolate in a double boiler over medium heat. Remove and set aside to cool to approximately room temperature.
In a large bowl combine butter and sugar. Mix on medium speed until butter is incorporated into sugar and it is the texture of wet sand.
Add in melted chocolate and mix thoroughly. Next add eggs and egg yolks. Mix until just combined.
Next add vegetable oil, vanilla, and sour cream. Mix thoroughly.
In a medium bowl combine dry ingredients- flour, salt, baking powder, and cocoa. Whisk to combine and break up any lumps.
Pour about ½ of the dry ingredients into the batter. Mix until just combined.
(If using coffee or espresso powder, mix into water)
Add ½ of the water and mix.
Repeat until no ingredients remain.
Fill cupcake liners until they are about ¾ of the way full.
Bake for 19-21 minutes, or until toothpick inserted into the center comes out clean.
Remove from cupcake tin to a wire rack to cool.