1. Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan (or springform pan) generously with butter or cooking spray. (Lining the bottom with parchment paper is recommended for easy removal).

2. Prepare the Topping: In a small bowl, whisk together the ⅓ cup sugar and 1 ½ teaspoons cinnamon. Set aside.

3. Mix Wet Ingredients: In a large bowl (or stand mixer), cream the softened butter, ¾ cup granulated sugar, and brown sugar together on medium speed until light and fluffy (about 3 minutes).

4. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

5. Add Dry & Liquid: In a separate small bowl, whisk the gluten-free flour, baking powder, and salt. Add half of this flour mixture to the butter mixture and mix gently. Pour in the milk and mix. Add the remaining flour mixture and mix just until combined. (Do not overmix).

6. Fold in Apples: Using a spatula, gently fold the diced apples into the batter until evenly distributed.

7. Fill the Pan: Scrape the batter into the prepared cake pan and smooth the top with your spatula.

8. Top It Off: Sprinkle the reserved cinnamon-sugar mixture evenly over the entire surface of the batter.

9. Bake: Bake for 40–50 minutes. The cake is done when the top is golden and crusty, and a toothpick inserted into the center comes out clean or with just a few moist crumbs (try to avoid hitting an apple chunk when testing).

10. Cool: Let the cake cool in the pan for at least 15-20 minutes. Run a knife around the edge to loosen it before releasing it from the pan (if using springform) or slicing directly from the pan.