In a large mixing bowl, combine the fresh berries with sugar, cornstarch, and vanilla extract. Gently stir until the berries are coated and the mixture is evenly combined. Set aside to allow the berries to macerate and release their juices.
Preheat your oven to 375°F (190°C). Spread the berry mixture into a baking dish, creating an even layer. Dot the top with small pieces of cold butter to help create a crunchy topping later.
In a separate bowl, combine flour and brown sugar. Add the cold butter and use your fingers or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs. This will be your streusel topping.
Sprinkle the crumbly streusel evenly over the berry filling, gently pressing it down to adhere.
Bake in the preheated oven for about 20 minutes, or until the topping is golden brown and the filling is bubbling at the edges. The aroma of baked berries and toasty streusel will fill your kitchen.
Remove from the oven and let cool slightly. The filling will thicken as it cools, and the topping will become crisp. Serve warm or at room temperature to enjoy the full burst of flavors and textures.
