Blueberry Rhubarb Shortbread Bars

Preheat oven to 325ºF. Grease a 9-inch square baking dish with nonstick cooking spray and set aside.

Sift flour into a medium bowl and set aside. In the bowl of an electric mixer, beat butter, powdered sugar and granulated sugar on high speed until light and fluffy; mix in vanilla until will combined. Stir in flour just until combined.

Press half of the dough into prepared dish and bake for about 15 minutes or until edges are golden brown; shape remaining dough into a disk and wrap tightly with Saran wrap; chill in refrigerator for 30 minutes.

When bottom layer is done baking, set aside to cool and prepare filling. Combine rhubarb, water, granulated sugar and cornstarch in a heavy duty saucepan over medium heat; stir occasionally and let mixture cook until fruit has formed a thick jam-like consistency (5-10 minutes). Remove from heat and fold in blueberries.

Pour fruit filling over crust. Remove remainder of dough from refrigerator and use all of it by dropping large crumbs over fruit layer so that the layer is completely covered. Place back into the oven and bake for about 30 minutes or until edges are very lightly golden brown.

Remove from oven and allow to cool before cutting into bars and serving. Store at room temperature up to 2 days.