For the cake batter: Preheat the oven to 375 degrees F. Line two 12-cup muffin pans with cupcake liners and set aside.
In the bowl of a stand mixer with a paddle attachment, add the sugar, gluten-free baking mix, cocoa powder and salt and mix on low speed until combined. Add the eggs one at a time, waiting until each egg is fully incorporated before adding the next. Add the milk, oil and vanilla and mix on low speed until combined. Stop the mixer and scrape down the sides of the bowl to make sure all the ingredients are thoroughly combined. Turn the mixer to low and slowly pour in 1 cup hot water, making sure it is evenly incorporated into the batter.
Fill the cupcake liners two-thirds full with the batter and bake until the top of each cupcake is slightly springy to the touch and a toothpick inserted in the center comes out clean, 14 to 16 minutes. Immediately turn the cupcakes out of the pans onto a cooling rack and let cool completely.
For the mint chocolate ganache filling: Put the cream in a saucepot and heat to a scald over medium heat. Do not boil!
Put the chocolate in a mixing bowl and pour the hot cream over it. Let it sit a moment and then gently stir to combine and melt the chocolate. Stir in the peppermint extract, and then set aside to cool and thicken up. The ganache can be made several days ahead of time and kept in an airtight container in the refrigerator.
For the mint chocolate chip Italian meringue buttercream: Put the sugar and 1/4 cup water in a heavy saucepan and heat over high heat until the sugar mixture registers 250 degrees F on a candy thermometer, 15 to 20 minutes.
While the sugar is cooking, whip the egg whites on medium speed in the bowl of a stand mixer to the soft-peak stage (the whites will look foamy and white), being careful not to over-whip. (If the whites start to look clumpy, they've been over-whipped. If this happens, toss them out and start again. If the whites are beating faster than the sugar is cooking, turn down the mixer speed. You can always turn up the speed if you need to.) Add the cream of tartar and continue whipping.
While the egg whites are whipping on medium speed, gently pour the sugar syrup down the side of the mixing bowl. Turn the speed up to high and whip until the mixing bowl no longer feels warm to the touch, about 7 minutes. Turn the mixer back to medium and add the butter in small chunks, whipping until it is completely incorporated. Stop the mixer, scrape down the sides of the bowl and add the peppermint extract and sea salt. Turn the mixer back to medium-low speed and mix until the peppermint extract is fully incorporated. Add the mini chocolate chips and stir to combine.
Fit an 18-inch piping bag with a round tip and twist and hold the bag right by the tip to prevent the buttercream from leaking out. Fold the top of the piping bag over your hand and spoon the buttercream into the bag.
To assemble, use an apple corer or knife to core out the center of each cupcake. Fill each cupcake with some of the mint chocolate ganache and frost the top of each with some mint chocolate chip Italian meringue buttercream.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.