Preheat the oven to 375°F (190°C) and grease an 8-inch baking pan.
In a mixing bowl, whisk together Bisquick mix, milk, sugar, and egg until smooth.
Pour the batter into the greased baking pan and spread evenly.
In a separate bowl, combine Bisquick mix, brown sugar, and cinnamon. Cut in the butter until the mixture resembles coarse crumbs.
Sprinkle the streusel topping evenly over the batter.
Bake for 18-22 minutes or until a toothpick inserted in the center comes out clean.
Let the cake cool for a few minutes before slicing and serving.
Video
Notes
For a nutty twist, add ½ cup of chopped pecans or walnuts to the streusel topping. Store leftovers in an airtight container at room temperature for up to 3 days.