Either grease with butter and flour, or spray with baking spray, an 9x9 inch pan, or place parchment paper on the bottom of the pan.
Make The Crust And Crumb Topping
In a medium bowl, stir together white sugar, flour, baking powder and salt.
In a measuring cup or small bowl, beat an egg with a fork. Add the egg to the flour mixture.
Using a fork or a pastry cutter, cut the butter into the flour mixture. Work with the dough until the pieces are very small. The dough will be crumbly.
Bake the Crust
Pat half of the dough evenly into the bottom of the prepared pan. Bake in a preheated oven for about 15 minutes or until it is lightly golden on the edges.
Make The Lemon Filling
While the crust is baking, prepare the lemon filling. In a large bowl, add the sugar, lemon zest, lemon juice, eggs, flour, salt, and baking powder.
Bake Crust And Filling
Pour the filling into a warm crust. Bake for about 15 minutes. Remove from oven.
Add Crumb Topping And Bake Some More
Add remainder of crumbly dough to the top of the lemon bars and return to the oven and bake for about another 15 minutes, or until the filling is firm, and the topping is a light golden brown.