Preheat oven to 350°F (175°C). Grease and line an 8x8 inch baking pan with parchment paper.

Make the crust: In a large bowl, combine flour, sugar, and salt. Cut in cold butter using a pastry blender or fork until the mixture resembles coarse crumbs.

Press crust into the pan: Press the crumb mixture firmly into the bottom of the prepared pan.

Bake the crust: Bake for 15-20 minutes, or until lightly golden brown.

Make the lemon filling: In a separate bowl, whisk together eggs, sugar, lemon juice, lemon zest, melted butter, baking powder, and salt.

Pour filling over crust: Pour the lemon filling evenly over the pre-baked crust.

Make the crumble topping: In the same bowl, combine flour, sugar, and cold butter. Cut in the butter using a pastry blender or fork until the mixture resembles coarse crumbs.

Sprinkle crumble over filling: Sprinkle the crumble topping evenly over the lemon filling.

Bake the assembled bars: Bake for 25-30 minutes, or until the topping is golden brown and the filling is set.

Cool completely: Let the bars cool completely in the pan before cutting into squares.

Lift and slice: Use the parchment paper overhang to lift the bars out of the pan. Cut into squares and serve!