For the cupcakes, preheat oven to 350° F. Grease or paper-line 20 muffin tin cups. Set aside.
Combine chocolate and cocoa powder in a small bowl. Carefully pour in boiling water and stir until the chocolate has melted and the ingredients are well combined. Add sour cream, cream de cacao, and vanilla extract. Mix well and set aside.
Combine flour, baking soda, and salt in a bowl and set aside.
In the bowl of a standing electric mixer, cream together butter and sugar until well combined.
Add the eggs to the butter mixture one at a time, beating well after each addition. Beat until well blended.
Add half of the flour mixture and mix briefly. Stir in the chocolate mixture. Add remaining flour and stir until just combined. Mix in chocolate chips.
Add batter to prepared muffin cups, filling each one about half full.
Bake until the cupcakes are set and a tester inserted in the center comes out mostly clean, about 15 minutes.
Remove from the oven and cool in the pan for 10 minutes before removing cupcakes to a cooling rack.
For the buttercream frosting, combine butter, 4 cups of confectioners' sugar, creme de menthe, and peppermint extract (if using) in the bowl of a standing electric mixer. Mix slowly with the paddle attachment to combine, and then increase speed to medium-high. Beat until well combined. Add additional powdered sugar or some of the milk or cream, as needed, to achieve a smooth consistency that will hold up on your cupcakes.
Pipe frosting atop cooled cupcakes or use a small spatula to add a generous layer of frosting to each cupcake. Garnish with sprinkles, if desired.