Preheat the oven to 350°F. Grease and flour an 8-inch square baking pan and set aside.
In a large bowl, whisk together the melted butter and sugar until combined. Add the eggs and vanilla, whisking until smooth.
Add the flour, baking powder, and salt, and mix until just combined. Add the milk and mix until smooth.
Spread the batter into the prepared pan. Sprinkle the blueberries over the top and set aside.
In a small bowl, mix together the brown sugar, flour, and cinnamon. Cut in the butter with a pastry blender or two knives until the mixture is crumbly.
Sprinkle the crumb topping over the blueberries. Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
Serve warm or at room temperature.
Interesting Facts
This crumb cake can be stored in an airtight container at room temperature for up to 3 days.
If using frozen blueberries, do not thaw them before adding to the cake.
This cake can be served as a dessert or a breakfast treat.