Heat oven to 350 F.

In large bowl, mix strawberries, blackberries and blueberries then add lemon juice and cornstarch; mix to combine. Refrigerate 20 minutes.

In medium bowl, whisk flour, baking powder and salt. Set aside.

In bowl of stand mixer, beat butter, brown sugar and vanilla extract on medium speed until blended. Gradually add flour mixture until crumbles form.

Spray 9-by-13-inch baking dish with nonstick cooking spray.

Pour berries into bottom of dish. Top berries with crumbles.

Bake 30-40 minutes, or until lightly browned.

Serve with ice cream.