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

In a mixing bowl, whisk together flour, sugar, baking powder, baking soda, and salt.

In another bowl, cream the softened butter until light and fluffy. Add the egg and mix well.

Gradually add the dry ingredients to the butter mixture, alternating with milk, and mix until just combined.

Gently fold in the blueberries.

In a separate bowl, combine flour, sugar, and cinnamon. Cut in the cold butter until the mixture resembles coarse crumbs.

Pour the cake batter into the prepared pan and spread evenly. Sprinkle the streusel topping over the batter.

Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted in the center comes out clean.

Allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.