Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, cream together the softened butter and sugar until light and fluffy.

Beat in the egg, then stir in the milk.

In another bowl, combine the flour, baking powder, and salt. Gradually add to the creamed mixture until just combined.

Fold in the blueberries gently.

Pour the batter into the prepared cake pan.

In a separate bowl, mix together the flour, sugar, and cinnamon.

Cut in the cold butter until the mixture resembles coarse crumbs.

Sprinkle the streusel topping evenly over the cake batter.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

Allow to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.