Prep: Let the sour cream and egg sit out on the counter and warm up while you prep the rest of the ingredients. They don't need to be room temperature, but just not super cold. Spray an 8-inch x 8-inch baking dish with cooking spray.

Make the crumb topping by combining the flour, brown sugar, cinnamon, and salt in a small bowl. Whisk to combine. 

Pour the 3 tablespoons of melted butter over the dry ingredients and mix until big clumps form and the dry ingredients are evenly moistened. Set aside.

To make the cake, combine the flour, baking powder, baking soda, salt, and lemon zest in a large bowl. Whisk to combine. 

In a medium bowl, combine the sugar, sour cream, and butter. Whisk until smooth. Add the egg and vanilla extract and mix until well combined.

Add the blueberries to the dry ingredients and quickly toss to coat.

Pour the wet ingredients over the dry ingredients and gently fold the batter until no streaks of flour remain. Distribute the berries as evenly as possible.

Spread into a prepared 8-inch x 8-inch baking dish and top with the streusel crumbs. Clump any sandy crumbs at the bottom of the bowl in your hand to make large crumbles.

Bake the coffee cake in a 350-degree F oven for 50-60 minutes until a toothpick inserted comes out clean.

For the best flavor, allow the coffee cake to cool to room temperature before serving.