Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Combine flour, butter, and confectioners' sugar in a bowl. Stir in macadamia nuts. Press half of the mixture into a 1/2-inch-thick layer on the bottom of the prepared pan to make the crust.
Bake in the preheated oven until lightly browned, 15 to 20 minutes.
Stir mangoes, white sugar, 1/3 cup water, and lemon juice together in a saucepan. Cook over medium heat until mangoes reduce by 1/3, 7 to 10 minutes.
Combine cornstarch and 3 tablespoons water in a small bowl. Pour into the mango mixture; cook until thickened, 5 to 7 minutes. Cool briefly and pour over the crust. Sprinkle the remaining flour mixture on top.
Continue baking until lightly browned, 20 to 25 minutes. Cool until set, about 15 minutes. Cut into bars.