• Preheat your oven to 375°F. Grease and line an 8″ square pan and set it aside.

Prepare the crumble topping.

• In a medium-sized bowl, place the melted butter. Add the Bisquick mix, brown sugar, cinnamon, and salt, and mix to combine well. The mixture will be thick. Place the bowl in the refrigerator to chill for about 10 minutes or until firm (or the freezer for about 5 minutes).

Make the cake batter.

• In a large bowl, place the Bisquick mix and 2 tablespoons sugar and whisk to combine. Add the milk, butter, and egg, and mix until just combined. Transfer the cake batter to the prepared pan and smooth into an even layer.
• Place the apple slices in a single, even layer on top and press gently to adhere. Sprinkle the apple slices lightly with ground cinnamon.
• Break up the chilled crumble topping with the tines of a fork into irregular pieces. Scatter the pieces evenly on top of the apples, and press gently to adhere.

Bake.

• Place the pan in the center of the preheated oven.
• Bake until the apples are fork-tender, the crumble topping is very lightly golden brown, and a toothpick inserted in the center comes out with no more than a few moist crumbs attached (about 25 minutes).
• Allow the cake to cool for at least 10 minutes in the pan before transferring to a wire rack to cool completely. Slice and serve.