Preheat an oven to 350 F. Grease and flour a 9x13” baking pan and set it aside.
Prepare the topping by mixing together the Bisquick, brown sugar, and cinnamon. Cut in the butter then set it in the fridge until needed.
In a large bowl, whisk together the cake mixture with the Bisquick, sugar, egg, sour cream, milk, and vanilla until smooth. The batter will be sticky.
Spread about ½ the batter into the prepared pan. It will be a thin layer.
Add about half of the cinnamon topping, then add the remaining batter in small dollops on top. Carefully smooth the remaining batter over the cinnamon filling. Top with remaining crumble mixture.
Bake in the preheated oven for 40 minutes, or until a toothpick inserted in the center comes out clean. Then let the cake cool.
Make the glaze by mixing the heavy cream with the powdered sugar until smooth, and then drizzle over the cooled cake.
Allow the glaze to set for 15 minutes before serving the cake.
Notes
A serrated knife is best for slicing. It’s great for cutting through the crumbly streusel topping without dragging it through the cake.
Spoon the Bisquick mix into a measuring cup without packing it down, then level off the excess with the flat edge of a knife to get the perfect amount.
Use cold butter for the streusel topping to give it the best crumbly texture.
Let the cake cool before adding the glaze. Otherwise, the glaze will melt and separate.
To store leftover coffee cake, let it cool completely, then transfer the remaining slices to an airtight container or cover tightly with plastic wrap. It will keep for 1-2 days at room temperature, 7 days in the fridge, or up to 3 months in the freezer.
This nutrition information is based on the exact products I used in this recipe. Brands and sizes of products could alter exact nutrition and should always be calculated independently.