Preheat oven to 350°F. Line a muffin tin with cupcake liners. In a large bowl, whisk together flour, cocoa powder, baking soda, and salt. In another bowl, cream butter and sugar until light and fluffy. Add eggs one at a time, mixing well. Stir in vanilla. Alternately add dry ingredients and buttermilk to the butter mixture, beginning and ending with dry ingredients.

Fill cupcake liners two-thirds full with batter. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

While cupcakes cool, whip heavy cream to soft peaks. Add powdered sugar and vanilla, and continue whipping until stiff peaks form. If using, fold in mascarpone cheese. Transfer to a piping bag fitted with a round tip.

Once cupcakes are cool, use a small knife to cut a cone-shaped piece from the center of each cupcake. Fill the cavity with cream filling and replace the top piece of cake.

If using ganache, heat heavy cream in a small saucepan until steaming. Pour over chopped chocolate in a bowl. Let sit for 5 minutes, then stir until smooth. Allow to cool slightly before using.

Pipe or spoon ganache over each filled cupcake, if desired. Allow ganache to set before serving.

Serve cupcakes at room temperature for the best texture and flavor. Pair with coffee, milk, or tea for a complete treat.

Store cupcakes in an airtight container in the refrigerator for up to 3 days. Allow to come to room temperature before serving.