Preheat the oven to 175°C and line a cupcake pan with paper liners.
In a small bowl, whisk together the cocoa powder and hot water until smooth, add the melted butter, sugar, and vanilla extract to this mixture then add the eggs, one at a time and mix until combined and smooth.
To the wet mixture add the flour, baking soda, baking powder, and salt and beat together until combined then blend in the sour cream.
Divide the batter between cupcake liners, filling them about 3/4 of the way full. Bake 18-20 minutes or until a toothpick inserted in the center comes out clean. Cool before filling and frosting.
Make the cherry filling: Place the cherries in a saucepan and add just enough water to cover, then add the sugar and cornstarch to the pot. Heat over medium-high heat, whisking occasionally, until the mixture begins to bubble and thicken. When it is thick, cover and chill.
To fill the cupcakes, use the cone method to remove a portion from the center of each cupcake. Fill each with a spoonful of the cherry filling.
Make the frosting: Using a hand mixer, stand mixer or whisk beat the mascarpone and powdered sugar together until light and fluffy. Slowly add the whipping cream until the frosting becomes light and fluffy.
Place the frosting in a piping bag and pipe onto the cupcakes. Garnish with cherries, chocolate shavings, or sprigs of mint.