Preheat the oven to 350°F. Butter or grease a 9-inch cake pan and line it with parchment paper.
In a small bowl, whisk together the 1 cup gluten free flour, 1 teaspoon baking powder, and ¼ teaspoon fine sea salt.
In a large bowl, using an electric mixer, cream the ½ cup unsalted butter and ⅔ cup granulated sugar until light and fluffy, about 3 minutes. Add the 2 large eggs and beat well to combine, scraping down the sides of the bowl. Beat in the 1 teaspoon pure vanilla extract and 3 tablespoons bourbon or rum. Mix in the flour mixture on low speed, then fold in the chopped 2-3 apples.
Pour the batter into the prepared pan use a rubber spatula to smooth the top. Sprinkle the surface evenly with 1 tablespoon of granulated sugar if you wish. Let the batter rest for 10 minutes, then bake for about 45 minutes, until a tester inserted into the center comes out clean.
Cool the cake in the pan set on a wire rack for 20 minutes, then turn it out gently to cool completely. Sift optional confectioners' sugar over the top of the cake to finish if desired.
Notes
CN's apple cake tips
Alcohol substitute. As mentioned, if you don't drink or don't happen to have any bourbon or rum in the house, no worries. You can leave it out or try these substitutions: 2 tablespoons vanilla extract, ½ teaspoon almond extract, or 3 tablespoons of tea.
Cool completely. This is a delicate cake, even more so than other gluten free bakes, because it doesn't call for a lot of flour. Make sure to cool the cake completely (at least 30 minutes) before slicing it so that the crumb has time to set and firm up a bit.
Try brown sugar. For a lovely hint of caramel complement to the apples, use light brown sugar instead of the granulated sugar.
Dairy-free. Substitute ½ cup of non-dairy butter for the butter to easily turn this into a dairy-free cake.
Add almonds. I love scattering sliced almonds over the top of the cake before baking for a bit of nutty crunch.
A little spice. This may buck tradition, but you can try adding ½-1 teaspoon of ground cinnamon or ginger to add a touch of fall-inspired spice to the cake.