Preheat oven to 375°F (190°C). Line a 9x13 inch pan with parchment paper, leaving an overhang.

In a large bowl, whisk together 3 cups flour, granulated sugar, baking powder, and salt.

Cut in the cold, cubed butter using a pastry blender until the mixture resembles coarse crumbs.

Lightly beat the egg and vanilla extract together, then toss into the crumb mixture until it holds together when squeezed.

Press slightly more than half of the dough firmly into the bottom of the prepared pan. Set the rest aside.

In another bowl, toss the diced apples with brown sugar, 1 tablespoon flour, cinnamon, nutmeg, lemon juice, and vanilla until coated.

Spread the apple mixture evenly over the bottom crust.

Sprinkle the reserved dough evenly over the apples, squeezing handfuls to create chunky streusel clumps.

Bake for 40-45 minutes until the top is golden brown and the filling is bubbly.

Cool completely in the pan on a wire rack before using the parchment sling to lift and cut into squares.