To make and bake the cupcakes:
Prepare muffin tins with 24 cupcake liners. Preheat oven to 350 degrees F with rack in the middle of oven.
Sift together the cake flour, baking powder and salt into a bowl, set aside. In a large bowl, whisk together 1 cup sugar and oil. Add 1/3 of the flour and whisk to incorporate. Whisk in 1/2 the milk. Repeat with another 1/3 of the flour, and the remaining milk. Finish with the last of the flour, add the vanilla and mix until well combined.
Whip the egg whites in a clean bowl until foamy, then gradually sprinkle the remaining sugar over them and beat until eggs are soft peaks. You can see in my picture that they will not droop off the whip when lifted out of the egg foam, but they won't have a stiff peak either.
Gently fold the whites into the batter.
Divide the batter in to the cupcake liners.
Bake for 18 to 20 minutes or until just starting to turn golden on the edges and a tester comes out clean.
To make the ganache:
In a saucepan heat the cream to a simmer, turn off the heat and add the chocolate and butter. Swirl the pan to make sure the chocolate is totally covered by the hot cream and allow to sit for 2 minutes. Add the salt and mint, gently whisk until smooth.
Spoon 1 tablespoon of the ganache over the cupcakes and smooth it to the edge, without going over. Allow the cupcakes to sit in a cool spot so the ganache will harden slightly. If you are in a rush, refrigerate them for 10 minutes.
To make the icing:
In a saucepan whisk together the flour and half-and-half over medium heat.
Continue whisking until the mixture thickens.
Put the mixture into a container and cover the surface with plastic wrap. Refrigerate until just cooled to room temperature, about 20 minutes or 15 in the freezer. Cream the butter, sugar and salt until light and fluffy, about 5 minutes. Add the cooled flour/milk mixture and whip until perfectly smooth. Add the mint extract, creme de menthe and green food coloring.
Once the ganache has set, fill a pastry bag, fitted with a large star tip, with the icing and decorate the cupcakes.
Leave a bit of the ganache showing under the icing. Sprinkle with the green sprinkles.
*Sift the flour into the cup when measuring, not after it's measured. **This is sold as baker's sugar or you can make your own by pulsing sugar in a food processor. Using regular sugar may result in a grainy icing.