Preheat the oven to 350°F / 177°C / Gas Mark 4. Line cupcake pans with papers.
In a medium bowl, whisk cocoa and hot water together until smooth.
Let cool slightly, then whisk in sour cream and melted chocolate until combined.
In a separate bowl, sift together flour, baking soda, baking powder and salt; set aside.
Using an electric mixer, cream butter and sugar until light and fluffy, 3 to 4 minutes.
Add eggs, one at a time, beating to combine after each. Scrape down sides of bowl. Beat in vanilla.
With mixer on low speed, add flour mixture in two parts, alternating with the cocoa mixture and beginning and ending with the flour; beat until combined.
Using a scoop (I used a size 20 scoop) divide batter evenly into cupcake liners. Bake for 16 - 18 minutes, or until a toothpick inserted comes out clean.
Let cool completely before frosting.
Mint Buttercream
Using an electric mixer, beat all ingredients together on low speed for one minute until combined, then at high for another 3 - 4 minutes until light and fluffy.
Transfer into piping bag fitted with star tip & pipe frosting onto cupcakes. Garnish with shaved creme de menthe candy.
Notes
To make this recipe kid-friendly, omit the creme de menthe and replace with a drop of green food coloring and a teaspoon of peppermint extract.