Make the cupcakes

1. Preheat the oven to 350 F (180 C).
2. Line muffin tins with 18 cupcake papers.
3. Sift the flour, cocoa, baking soda, and salt into a large bowl.  Do not just whisk the ingredients together as the cocoa often has lumps that will be hard for a whisk or just your hands to break up.  If you don’t have a sifter, press the ingredients through a sieve into the bowl.
4. Microwave the chocolate and butter in a cup.  It should take about 40-50 seconds to melt completely.
5. Mix the melted butter, melted chocolate, sugar, and mascarpone in a mixing bowl.  Use either a hand mixer or a stand mixer to whip everything until smooth.  Add in the eggs and vanilla and mix until smooth.
6. Make a well in the dry ingredients and then scrape in the wet ingredients.  Pour the Guinness over the wet ingredients and the additional chocolate chips if you’re using them.  Use a spatula to mix everything together just until the flour disappears.
7. Fill the cupcakes with about 1/4 cup of batter.
8. Bake the cupcakes for 18-20 minutes until the tops spring back in the middle when touched.
9. Turn out the cupcakes onto a cooling rack to cool for 10 minutes.

Frost the cupcakes

1. Fit a pastry bag with a decorative star tip.  Dip a small paintbrush in the gel coloring and paint stripes down the sides of the bag at intervals.  4-6 stripes will make a nice green-tinged whipped cream.  Set the bag inside a tall cup.
2. Melt the marshmallow in the microwave until poofy but not hard—about 25-30 seconds.  Immediately scrape out the marshmallow into a mixing bowl with the heavy cream, Irish cream, and the powdered sugar.  Whip the cream with your mixer until it holds its shape and no longer drips off the end of the whip attachment.
3. Fill the pastry bag with the cream, pressing lightly on the sides of the bag to help the food coloring stick to the cream.
4. Pipe a swirl of cream on top each cupcake.
5. Cupcakes are best eaten the day they’re made, but you can store leftovers in a covered container in the fridge for up to 3 days.