Preheat the oven to 375°F and line an 8x8-inch baking pan with parchment paper and set aside.
In a large mixing bowl, beat together the bisquick, milk, sugar, eggs, vanilla, and salt until combined and smooth.
Pour the batter into the prepared baking dish and set aside.
In a medium microwave-safe mixing bowl, melt the butter, then stir in the Bisquick, brown sugar, and cinnamon and mix with a rubber spatula or your hands until combined and large crumb balls for.
Sprinkle the crumb topping over the top of the batter then bake for 35 to 40 minutes or until a toothpick comes clean from the center of the cake.
Cool in or out of the pan. But I recommend allowing it to cool completely before cutting.
Notes
You can double this recipe and bake in a 9x13-inch pan.
Top this with some powdered sugar or a simple powdered sugar glaze.