Preheat your oven to 350°F (175°C) and line a 9x13-inch baking pan with parchment paper.

In a large mixing bowl, whisk together flour, granulated sugar, light brown sugar, baking powder, and salt. Cut in cold butter until the mixture resembles coarse crumbs. Add egg and vanilla extract, mixing until a soft dough forms.

Press approximately ⅔ of the dough into the bottom of the lined baking pan to create the crust base.

In a large bowl, toss sliced apples with granulated sugar, light brown sugar, cornstarch, cinnamon, nutmeg, and lemon juice until well-coated.

Spread the apple mixture evenly over the crust in the pan. Crumble the remaining dough over the top.

Bake in the preheated oven for 45-50 minutes until golden brown and bubbling.

Remove from oven and let cool completely in the pan before slicing.

For the glaze, whisk powdered sugar, milk, and vanilla extract in a bowl and drizzle over cooled slices.