Preheat your oven to 375°F (190°C). Grease an 8-inch square baking pan or line with parchment paper.
For the cake, in a large bowl whisk together the Bisquick, milk, sour cream, sugar, and egg until smooth.
Fold in the blueberries.
Pour the batter into the prepared pan.
To make the cinnamon topping, in a small bowl mix together the brown sugar, Bisquick, cold butter pieces, and ground cinnamon. Cut the cold butter into the mixture with a fork until crumbly.
Sprinkle the topping mixture over the top of the cake batter.
Bake for 25-30 minutes, or until a toothpick inserted into the center of the cake comes out with a few moist crumbs.
Allow the cake to cool for at least 10 minutes before serving. Serve warm or cool to room temperature.
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Notes
Preparing the baking dish: If you line the baking dish with parchment paper, you can lift the baked cake out of the pan to make it easier to cut on a flat surface. Or, you can cut it directly in the baking dish.What is folding in? This is a technique used in baking that involves gently mixing ingredients without deflating the batter. When a recipe calls for folding in an ingredient, it typically means to add it to the batter using a spatula or wooden spoon and then gently turning the batter over itself until the ingredient is evenly distributed.