How to Make It
Preheat oven to 375°F. Line an 8-inch square pan with foil, leaving a 2-inch overhang on 2 sides. Mist with cooking spray.
Make crumb mixture: Using an electric mixer on medium-low speed, combine butter, flour, salt, vanilla and 1 cup sugar until mixture resembles coarse crumbs. Spread 1/2 of mixture evenly over bottom of prepared pan and press down.
Make filling: Combine rhubarb, strawberries, lemon juice, cornstarch and remaining 1/4 cup sugar in a large bowl. Spread evenly over crust in pan. Sprinkle remaining crumb mixture over fruit. Bake until bubbly and browned, 45 to 55 minutes. Let pan cool completely on a wire rack.
Using foil overhang, carefully lift bars out, peel back foil and cut into 9 squares. Serve at room temperature.