Preheat oven to 350 degrees. Line an 8×8" baking dish with parchment paper or foil.

Prepare the crust. In a large bowl, beat the butter and 1/4 cup powdered sugar till light and fluffy, about 2-3 minutes. Add flour and salt, and beat on low speed until just incorporated.

Scrape the dough into the prepared pan and spread evenly with an offset spatula. Bake on the middle rack of the oven 20-25 minutes, or until lightly golden. Remove from the oven and cool on a wire rack while you prepare the filling.

Prepare the filling. In a medium bowl, whisk the eggs and granulated sugar until well incorporated. Add the 3 tablespoons of flour and whisk until combined, then whisk in the lime juice and zest.

Pour the filling over the cooled crust, and bake 20-25 minutes longer, or until the filling has set. The filling will not "wiggle" when the pan is moved gently back and forth. Remove from heat and cool completely on a wire rack.

Once the key lime bars have cooled, cut into squares with a sharp knife, wiping the knife clean in between slices. Dust with powdered sugar, serve and enjoy!