Step 1: Make your filling
6 TBS butter, at room temp.
3/4 cup marshmallow fluff
1.5 cups powdered sugar
2 TBS milk
Step 2: Fill your pastry bag, outfitted with a star tip, as if this was frosting. Be aware you will likely have some oozing as this filling is runnier than traditional frosting.
Step 3: Push the tip into the center of your cupcakes and add filling until you just start to feel the cupcake bulge out
You may have some seepage out of the hole, but that is ok.
Remember, this is going to be covered with frosting yet
When cut in half you can see how full of yummy creamy goodness these cupcakes really are.
Happy St. Patrick's Day!