Start by preheating your oven to 350°F (175°C). This ensures that your cake bakes evenly.
Grease a 9-inch round baking pan with butter or non-stick spray. If you prefer, line the bottom with parchment paper for easy removal.
In a large mixing bowl, combine the all-purpose flour, baking powder, brown sugar, and kosher salt. Whisk these ingredients together until well blended.
Gently fold the vanilla cake mix into the dry ingredients until combined. Be careful not to overmix; a few lumps are perfectly fine.
Gently fold the fresh blueberries into the batter. This will ensure they’re evenly distributed throughout the cake.
In a separate bowl, mix together 1/3 cup of the prepared batter with an additional tablespoon of melted butter, and a pinch of brown sugar. Use your fingers to crumble the mixture together until it resembles coarse crumbs.
Pour the blueberry batter into the greased baking pan. Use a spatula to spread it evenly.
Sprinkle the crumb topping evenly over the batter. This will create a deliciously buttery crunch when baked.
Place the pan in the preheated oven and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Keep an eye on the cake as oven temperatures can vary.
Once baked, let the cake cool in the pan for about 10 minutes before transferring it to a wire rack. Serve warm or at room temperature, and enjoy the delightful flavors of your Blueberry Coffee Cake with Crumb Topping.
