Coconut Lemon Bars with Marshmallow Frosting

Heat oven to 375 degrees F. Coat an 8-inch square pan with cooking spray. Line the pan with parchment paper, leaving a 3-inch overhang on two sides. Spray the paper. Make the crust: In a medium bowl, beat the egg whites and sugar until frothy; mix in the coconut. Spread the mixture into the prepared pan and bake until golden brown, 15 to 20 minutes. Meanwhile, make the lemon curd: In a large bowl, whisk together the sugar, flour, cornstarch, and salt. Add the egg yolks, lemon zest, and juice and whisk to combine. Spread over the crust and bake until set, 25 to 35 minutes. Let cool to room temperature, then refrigerate until chilled, at least 3 hours or up to overnight. Just before serving, make the marshmallow frosting: In a large metal or glass bowl, whisk together the reserved egg whites, sugar, and cream of tartar. Set the bowl over (but not in) a saucepan of simmering water and cook, whisking constantly, until the sugar is dissolved and the whites are very warm to the touch, 3 to 4 minutes. Remove from heat and, using an electric mixer, beat on low speed, gradually increasing to high, until soft, glossy peaks form, about 5 minutes. Beat in the vanilla. Spoon the frosting over the lemon curd, gently spreading to cover. Using the overhangs, transfer to a cutting board and cut into 2-inch squares.