Start by preheating your oven to 350 degrees Fahrenheit. Grease a 9-inch square or round baking pan thoroughly with butter or a light coating of oil, then line the bottom with parchment paper to ensure the cake releases perfectly after it finishes baking.
In a large mixing bowl, whisk together the gluten-free flour blend, granulated sugar, light brown sugar, baking powder, salt, and the tablespoon of cinnamon. Ensure there are no large clumps of brown sugar so the batter remains smooth and consistent.
In a separate medium bowl, beat the softened butter until it is creamy. Add the eggs one at a time, followed by the whole milk and the alcohol-free vanilla extract. Whisk these together until they are well combined, though the mixture may look slightly curdled due to the butter, which is perfectly normal.
Slowly pour the wet ingredients into the dry flour mixture. Use a spatula or a wooden spoon to stir everything together until just combined. Be careful not to overmix, as you want the cake to remain light and fluffy rather than dense.
Gently fold in the peeled and diced apples until they are evenly distributed throughout the batter. Spread the batter into your prepared pan. In a small bowl, mix the topping sugar and cinnamon together, then sprinkle it generously over the entire surface of the cake batter.
Place the pan in the center of the oven and bake for 40 to 45 minutes. You will know it is ready when a toothpick inserted into the center comes out clean. Let the cake cool in the pan for at least 15 minutes before slicing so the structure can set properly.
