Rhubarb Squares

Steps

1. Preheat oven to 350°F. In a medium bowl, combine 2 cups flour, 1½ sticks butter, and confectioners’ sugar. Cut butter into dry ingredients with two butter knives or a pastry knife. Mix until a crumbly texture. Press crumb mixture into a 10x15" jelly roll pan. Bake 10 minutes. Remove pan from oven and increase oven to 375°F.
2. Meanwhile, in a medium pot on medium, combine rhubarb and granulated sugar. Cook 13 to 14 minutes, until rhubarb is soft but still holds its shape. If desired, purée rhubarb with a handheld immersion blender. Let cool.
3. Melt remaining 2 tablespoons butter in microwave-safe bowl. In a medium bowl, combine melted butter, juice of both lemons, remaining 2 tablespoons flour and egg yolks. Stir in cooled rhubarb mixture.
4. Using a stand mixer or hand mixer, beat egg whites on high 1 to 2 minutes, until foamy. Beat egg whites into rhubarb mixture until just combined. Spread mixture onto crust. Bake until set, 30 to 35 minutes. Let cool completely. Dust with confectioner’s sugar. With a paring knife, cut tray into 9 rows. Cut each row into 1" squares.