For the Streusel:

1/3 cup granulated sugar

1/2 cup all-purpose flour

1 tsp cinnamon

1/8 tsp table salt

4 tbsp butter, softened

For the Cake:

2 cups all-purpose flour

2 tsp baking powder

1/2 tsp table salt

3/4 cup granulated sugar

4 tbsp butter, softened

1 large egg, room temperature

1 tsp vanilla extract

1/2 cup milk, room temperature

2 cups blueberries, fresh or frozen

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Preheat oven to 375°F (190°C). Lightly grease a 9-inch square or round cake pan.
Make the streusel: Mix sugar, flour, cinnamon, and salt. Work in butter with a fork or fingertips until crumbly. Set aside.
Make the cake: Whisk flour, baking powder, and salt in a bowl.
In a separate bowl or stand mixer, beat sugar, butter, egg, and vanilla until combined.
Add milk and flour mixture alternately, ending with flour.
Gently fold in blueberries just until combined.
Spread batter into the prepared pan and sprinkle streusel evenly over the top.
Bake 40–45 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the pan for 10 minutes before serving or transferring to a plate.

Add lemon zest to the batter for citrus brightness.

Swap cinnamon for cardamom for a different flavor.

Fold in chopped pecans or walnuts for crunch.

Use mixed berries for a colorful twist.

Drizzle a simple vanilla glaze for extra sweetness.