Preheat the Oven:
Preheat your oven to 375°F (190°C). Grease and lightly flour a 9-inch baking pan or line it with parchment paper.
Prepare the Streusel Topping:
In a bowl, mix together flour, brown sugar, and cinnamon. Cut in the cold butter using a fork or pastry cutter until the mixture resembles coarse crumbs. Set aside.
Mix the Dry Ingredients:
In a medium bowl, whisk together flour, baking powder, and salt.
Cream Butter and Sugar:
In a large bowl, beat the softened butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Mix in the vanilla extract.
Combine the Wet and Dry Ingredients:
Gradually add the dry ingredients to the wet mixture, alternating with the milk. Begin and end with the dry ingredients. Mix until just combined.
Add Blueberries:
Gently fold in the blueberries, ensuring they are evenly distributed.
Assemble the Cake:
Pour the batter into the prepared pan, spreading it evenly. Sprinkle the streusel topping generously over the batter.
Bake:
Bake for 35–40 minutes or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes before transferring it to a wire rack.