Follow the instructions according to the recipe and allow the cupcakes to cool completely. Core the cupcakes using a cupcake corer, apple corer, melon baller, or small knife. Take out a little extra cake with your fingers. Keep the small top of the cupcake ("plug") to seal the cupcake back up after filling.
FOR THE FILLING
Whip all the ingredients for the filling in the bowl of a stand mixer using the whisk attachment until smooth and fluffy, about 5 minutes on high. Fill a pastry bag or ziptop bag with the filling and pipe into the cored cupcakes. Replace the "plug" of cupcake on top of the filling.
FOR THE CHOCOLATE FUDGE ICING
In a small saucepan, mix sugar, cocoa powder, and milk together over medium heat. Whisk often until the sugar dissolves. Increase the heat to medium high and bring the mixture to a full hard (rolling) boil, whisking constantly. Boil for one minute. Remove from the heat and add the cubed butter and vanilla extract and whisk until smooth.
Allow the icing to cool down until it thickens up enough to spoon over the top of each cupcake. It should be the consistency of a thick sauce, such as a hollandaise or a hot fudge.
FOR THE SQUIGGLY WRITING ON TOP
Fill a pastry bag (or ziptop bag) with a Wilton #4 tip and white royal icing and pipe either continuous loops or any words you'd like to say on the cupcakes. Allow to dry completely before serving.
Cupcakes will keep at room temperature for a few days. They can also be frozen, well wrapped, for up to one month. Thaw, still wrapped, at room temperature for a few hours.