For the Cake

¾ cup (105 g) gluten-free cake flour (86 g high-quality all-purpose gluten-free flour + 19 g cornstarch)*
¼ teaspoon xanthan gum (omit if your blend already contains it)
½ teaspoon baking powder
½ teaspoon baking soda
¼ teaspoon kosher salt
1 cup (200 g) granulated sugar
6 tablespoons (84 g) unsalted butter, at room temperature
3 extra-large eggs (180 g, without the shell) at room temperature, beaten
1 tablespoon pure vanilla extract
3 to 4 tablespoons warm water
Confectioners' sugar, for sprinkling

*In place of 3/4 cup gluten-free cake flour, you can use the same amount, by weight, high-quality all-purpose gluten-free flour. Your cake will simply be a bit denser.

For the Chocolate Buttercream Filling

6 tablespoons (84 g) unsalted butter, at room temperature
7 tablespoons (84 g) vegetable shortening
4 ounces semi-sweet chocolate, chopped
2 ½ to 3 cups (288 g to 345 g) confectioners' sugar
1 tablespoon light corn syrup

For the Ganache Glaze

12 ounces semi-sweet chocolate, chopped
½ cup heavy whipping cream