Preheat the oven to 375 degrees F (190 degrees C). Spray an 8-inch or 9-inch square baking dish with nonstick cooking spray.

In a large bowl, gently stir the frozen blackberries together with 2 tablespoons sugar, lemon juice, and vanilla extract. Spread into the bottom of the prepared baking dish.

Combine flour, remaining 1 cup sugar, and egg together in a bowl until mixture resembles a coarse meal. Sprinkle mixture evenly over the blackberries. Drizzle melted butter evenly on top.

Bake in the preheated oven until lightly browned and bubbly, 35 to 40 minutes. Remove from the oven and let cool for 10 minutes. Serve warm topped with whipped topping.