Preheat the oven: Preheat your oven to 350°F (175°C). Grease and flour a 9×9-inch square baking pan or a similarly sized baking dish.

1. Mix dry ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
2. Cream the butter and sugar: In a large bowl, cream the softened butter and sugar together using an electric mixer on medium speed until light and fluffy, about 3-4 minutes.
3. Add the eggs and vanilla: Beat in the eggs one at a time, followed by the vanilla extract, and mix until combined.
4. Add the dry ingredients and milk: Gradually add the flour mixture to the wet ingredients, alternating with the milk. Begin and end with the dry ingredients, mixing until just combined. Be careful not to overmix.
5. Fold in the blueberries: Gently fold the fresh blueberries into the batter. Be careful not to break the berries too much, as you want them to stay intact in the cake.
6. Transfer to the pan: Pour the batter into the prepared baking pan and spread it evenly.
7. Make the streusel: In a small bowl, combine the flour, brown sugar, and cinnamon. Add the cold, cubed butter and use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture resembles coarse crumbs.
8. Top the cake: Sprinkle the streusel mixture evenly over the top of the cake batter.
9. Bake the cake: Place the pan in the preheated oven and bake for 45-50 minutes, or until a toothpick inserted into the center of the cake comes out clean. If the streusel topping starts to brown too quickly, cover it loosely with foil.
10. Cool: Allow the cake to cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely before slicing.
11. Serve: Slice into squares and serve! You can enjoy it on its own, or with a dollop of whipped cream or a scoop of vanilla ice cream for an extra indulgence.