Preheat the oven to 350°F/180°C, line a muffin pan with cupcake liners. In a bowl, combine cocoa powder with espresso powder, flour, baking soda, baking powder, and salt.
Pour the batter into cupcake liners ¾ full. Bake for 20 minutes. Remove from the oven, and allow to cool down completely. As you wait, make the chocolate buttercream.
To make the buttercream: in a large bowl using a hand mixer, or using a stand mixer that’s fitted with a whisk or a paddle attachment, beat the butter at medium speed for 1 minute until creamy.
When the sugar is mixed in, increase the speed to high and beat for 3 minutes. If the frosting is too thick, add cream and beat again. If it’s too thin, add powdered sugar and beat again.