Mix 1 cup flour and confectioners' sugar together in a bowl; cut butter into flour mixture using a pastry cutter or two forks until mixture is crumbly. Press flour-butter mixture into an 8-inch square pan.
Bake crust in the preheated oven until golden brown, 15 to 20 minutes.
Mix white sugar, 1/4 cup flour, baking powder, and nutmeg into the beaten eggs. Fold rhubarb into egg mixture; pour rhubarb filling into the baked crust.
Bake in the preheated oven until filling is bubbling, 35 minutes.
