Step 1: Prepare the Crust and Topping

1. Preheat your oven to 350°F (175°C). Grease and line a 9×13-inch baking pan with parchment paper, leaving an overhang for easy removal.
2. In a mixing bowl, whisk together flour, sugar, salt, and baking powder. Add the cold, cubed butter. Using a pastry cutter, fork, or your fingertips, work the butter into the flour mixture until it resembles coarse crumbs.
3. Press half of the mixture firmly into the bottom of the prepared pan to form the crust. Set the other half aside for the topping.
4. Bake the crust for 10-12 minutes, or until it’s just beginning to set but not browned. Remove from the oven and let cool slightly.

Step 2: Make the Lemon Filling

1. In another bowl, whisk together sugar, flour, and salt. Add eggs, lemon juice, and zest, whisking until smooth and well combined.
2. Pour the lemon filling over the partially baked crust, spreading it evenly.

Step 3: Add the Crumb Topping

1. Sprinkle the reserved crumb mixture evenly over the lemon filling. Gently press it down to adhere slightly.

Step 4: Bake and Cool

1. Bake the bars for 25-30 minutes, or until the topping is golden brown and the filling is set with a slight jiggle in the center.
2. Let the bars cool completely in the pan. Once cooled, use the parchment overhang to lift the bars out.

Step 5: Slice and Serve

1. Slice into squares or rectangles. Dust with powdered sugar for an extra touch if desired.