Preheat oven to 375 degrees. Use a pastry brush to grease 24 mini-sized muffin cups with softened butter. Dust with cocoa powder and tap out excess.
Melt both chocolates, butter and 1 tablespoon of the sugar for 1 minute in the microwave and then whisk until smooth (or heat over a pot of simmering water). Let stand until it cools and thickens slightly, about 5 minutes.
Add eggs, remaining 2 tablespoons of sugar, the flour and espresso powder to the bowl of a food processor and process for about 2 minutes. Add chocolate mixture in four additions, pulsing after each until combined (it is a thick batter).
Use a small cookie scoop (a scant tablespoon) to add dough into prepared muffin cups, leveling the tops with the back as needed.
Bake for 10 minutes. Cool in pans 5 minutes before turning out onto cooling racks to cool completely.
*I have found that both chips or chopped chocolate work equally well for this recipe, so use what you have.**You can use any flour, even almond flour to keep it GF.***The espresso powder is optional, though I do like how it subtly brings out the chocolate flavor more.