Preheat your oven to 375°F and grease a 9×9-inch baking dish.
Toss the frozen berries with sugar, cornstarch, lemon juice, lemon zest, and vanilla. Pour into the dish and spread evenly.
In another bowl, whisk the flour, sugars, baking powder, and salt. Stir in melted butter and vanilla until a soft cookie dough forms.
Crumble the dough evenly over the berry mixture.
Bake for 40–45 minutes or until the topping is golden and the filling is bubbly.
Let cool for 10 minutes, then serve warm with a scoop of vanilla ice cream or whipped cream (optional but HIGHLY recommended).
