Step 1: Preheat your oven to 350°F (175°C) to ensure that your cake bakes evenly.
Step 2: In a large mixing bowl, combine the all-purpose flour, baking powder, and kosher salt. In another bowl, whisk together the vanilla cake batter until smooth. Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
Step 3: Gently fold in the fresh blueberries, being careful not to overmix.
Step 4: In a separate bowl, mix the melted butter and brown sugar until combined. Add a pinch of salt and then gradually incorporate the flour until crumbly.
Step 5: Pour the cake batter into a greased 9-inch round cake pan, spreading it evenly. Sprinkle the crumb topping generously over the batter.
Step 6: Bake in the preheated oven for about 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Step 7: Once baked, let the cake cool in the pan for about 10 minutes, then transfer it to a cooling rack to cool completely. Slice and serve warm or at room temperature.