Line a 9 x 13-inch pan with aluminum foil and spray with PAM Cooking Spray.
Line a baking sheet with parchment. Set aside. Preheat the oven to 350º.
To make the dough, combine flour, sugar, baking powder, and salt in a bowl and stir to combine.
Add butter pieces and using your fingers pinch and rub the butter till it's totally worked into flour (no big pieces will be visible).
Mix eggs into the dough until well incorporated.
Knead gently in the bowl till the dough comes together.
On a floured surface roll dough to a 12 x 16-inch rectangle. Fit into the prepared 9 x 13 pan and trim so there's just a ½-inch edge going up the sides of the pan.
Put a rectangle of parchment on top of the bottom crust and weigh it down with dried beans or pie weights. Bake for 10 minutes. Remove parchment and beans and bake for 5 more minutes. Cool on rack.
To make the topping, combine flour, sugar, baking powder, and salt.
Add melted butter and combine with a rubber spatula.
Spread out on a baking sheet, breaking into small crumbs. Bake 15-20 minutes, till golden brown. Cool on rack. If necessary, break it into small crumbs when cool.
To make the filling, whisk eggs and zest in a bowl. Mix lemon juice and sugar in another bowl to dissolve sugar. Stir egg and juice mixtures together. Pour over the crust and bake for 15-20 minutes.
Sprinkle with the crumb topping and bake for an additional 5 minutes.
Cool completely on a wire rack, then cut into bars.
