Preheat oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a bowl, blend flour, sugar, baking powder, and salt. Add melted butter, milk, egg, and vanilla extract. Mix until combined.
Gently fold in the blueberries into the mixture and pour batter into the prepared cake pan.
For the streusel topping, add all-purpose flour, light brown sugar, and ground cinnamon in another bowl. Cut in cold butter until mixture is crumbly, then sprinkle over the batter.
Bake in preheated oven for 40 minutes, or until a toothpick inserted into the center comes out clean.
Allow cake to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.
Notes
For extra flavor, add a pinch of nutmeg to the batter or serve with a dollop of whipped cream.