Preheat oven to 400 degrees. Grease a 9x13 cake pan.
To prepare the cake, combine Bisquick, milk, egg, sugar, and ½ tsp cinnamon in a large mixing bowl until it forms a consistent batter.
Pour into the prepared pan. Set aside.
To prepare the topping, in a separate bowl, mix 1 cup of Bisquick, brown sugar, butter, and 1 tsp cinnamon in a medium bow.
Use a fork or pastry blender to work the butter into the mixture until crumbly.
Spread the brown sugar streusel evenly over the cake batter, and use a butter knife to make swirls across the top.
Transfer the baking pan to the preheated oven, and bake for 22-25 minutes, until golden brown and a toothpick inserted into the center comes out clean.
Allow the coffee cake to cool in the pan for about 10-15 minutes before transferring it to a wire rack to cool completely.
Store any leftover cake in an airtight container to keep it fresh.
Serve your Bisquick Coffee Cake warm or at room temperature with a dusting of powdered sugar or a dollop of whipped cream on top of the cake for a delightful finish.