Preheat the oven to 350 degrees Fahrenheit and line a 9-inch square baking pan with parchment paper.

In a large mixing bowl, whisk together 1 and 1/2 cups all-purpose flour, 3/4 cup granulated sugar, 2 teaspoons baking powder, and 1/2 teaspoon salt.

In a medium bowl, whisk together 6 tablespoons melted butter, 2 large eggs, 1/2 cup whole milk, and 1 teaspoon vanilla extract until smooth.

Pour the wet ingredients into the dry ingredients and gently stir with a rubber spatula until just combined, leaving a few small lumps.

Peel, core, and slice 3 medium Granny Smith apples into 1/4-inch thick slices.

Spread half of the batter into the prepared pan, arrange the apple slices in a single layer over the batter, then pour the remaining batter on top and smooth the surface.

In a small bowl, combine 1/2 cup flour, 1/2 cup packed brown sugar, 1 and 1/2 teaspoons ground cinnamon, and 1/4 teaspoon salt, then rub in 4 tablespoons cold cubed butter with your fingertips until coarse crumbs form.

Evenly sprinkle the streusel topping over the cake batter without pressing it down.

Bake for 35 minutes or until a toothpick inserted into the center comes out with moist crumbs and the streusel is golden brown, tenting with foil for the last 10 minutes if it browns too quickly.

Allow the cake to rest in the pan for 10 minutes, then lift out using the parchment overhang and cool completely on a wire rack before slicing and serving.