Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

Prepare the apple filling: In a large bowl, combine the diced apples, 1/4 cup granulated sugar, lemon juice, and 1 tsp cinnamon. Toss well to coat and set aside.

Make the cake batter: In a large mixing bowl, cream together the softened butter and 1 1/2 cups granulated sugar until light and fluffy.

Beat in the eggs one at a time, then stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined. Be careful not to overmix.

Create the crumb topping: In a medium bowl, combine the flour, sugar, and cinnamon for the crumb topping.

Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. The butter needs to be cold for the best crumbly texture.

Assemble the cake: Pour the cake batter into the prepared baking pan, spreading it evenly.

Spoon the apple filling over the cake batter, distributing it evenly.

Sprinkle the crumb topping evenly over the apple filling.

Bake in the preheated oven for 45-55 minutes, or until a wooden skewer inserted into the center comes out clean. If the crumb topping starts to brown too quickly, tent the cake loosely with foil during the last 15-20 minutes of baking.

Let the cake cool in the pan for at least 15 minutes before slicing and serving.