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

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

Add the softened butter and mix until crumbly.

Stir in the egg and milk until just combined. Gently fold in the blueberries.

Pour the batter into the prepared cake pan.

In a separate bowl, combine the flour, sugar, and cinnamon for the streusel.

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 before transferring to a wire rack to cool completely.