Sift together the flour, cocoa powder, espresso powder, baking powder, and salt in a medium bowl. Set aside.
Prepare the wet ingredients:
Combine the butter and brown sugar in a large bowl. Beat with an electric mixer until combined.
Add the eggs and beat until combined. Add the sour cream, milk, and vanilla extract and continue mixing until smooth.
Add the dry ingredients to the batter:
Add the bowl of dry ingredients to the batter and beat with an electric mixer at medium speed until combined. Don’t overwork the flour!
Bake the cupcakes:
Pour the batter into the liners, filling each one up about ⅔ of the way full. Transfer to the oven for 20 to 23 minutes or until a toothpick inserted into the cupcake comes out clean.
Transfer the cupcakes to a wire rack to cool completely, at least 1 hour.
Make the mint buttercream frosting:
Add 3½ cups powdered sugar, butter, mint extract, vanilla extract, green food coloring, and heavy cream to a large bowl and beat with an electric mixer until smooth, being careful not to overmix. Add more heavy cream as needed to reach a smooth, creamy consistency.
Fold in all but 1 tablespoon of the finely chopped dark chocolate until combined.
Frost the cupcakes:
Transfer the frosting to a piping bag fitted with an extra-large round tip. Pipe the frosting onto each cupcake.
To serve:
Sprinkle the cupcakes with the reserved finely chopped dark chocolate and sprinkles. Enjoy!
Notes
Note 1: Make sure you pulverize the dark chocolate as much as possible otherwise it will get stuck in the tip of the piping bag when you frost the cupcakes.