Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan, or line it with parchment paper.

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

Add the eggs one at a time, mixing well after each addition, then incorporate the vanilla extract.

In another bowl, whisk together the flour, baking powder, and salt.

Gradually add this dry mixture to the wet ingredients, alternating with the milk. Start and end with the flour mixture.

Gently fold in the blueberries, saving a few for the top.

In a medium bowl, combine the flour, brown sugar, melted butter, and cinnamon. Mix until crumbly.

Pour the cake batter into the prepared pan and smooth the top. Sprinkle the streusel evenly over the batter and add reserved blueberries.

Bake for 45-50 minutes or until a toothpick inserted into the center comes out clean.

Let the cake cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.