Preheat the oven to 350°F (175°C). Grease a 9×9-inch baking pan or line it with parchment for easy removal.
Make the crust: In a medium bowl, combine the flour and powdered sugar. Cut in the softened butter until the mixture resembles coarse crumbs. Press evenly into the bottom of the prepared pan.
Bake the crust for 15 minutes, until it’s set and just beginning to color. Remove from oven.
Filling
In a separate bowl, beat the granulated sugar and eggs until smooth. Add the lemon juice, lemon zest, and baking powder; whisk until fully combined.
Pour the filling over the hot crust, smoothing the top. Return the pan to the oven and bake for about 20 minutes, or until the filling is set but still has a slight jiggle in the center.
Cooling and Serving
Cool completely on a wire rack. Chill briefly in the fridge if you want cleaner slices. Dust with powdered sugar and cut into bars.
Notes
For a slightly sturdier crust, swap 2 tablespoons of the flour for almond flour. If you prefer a creamier filling, consider condensed milk variations. To make gluten-free, use a 1:1 gluten-free flour blend and press firmly when forming the crust.