Preheat your oven to 175°C (350°F) and line a 23×33 cm baking pan with parchment paper.

In a mixing bowl, cream the softened butter for 2–3 minutes until light and fluffy, then add granulated sugar and all-purpose flour, mixing until well combined.

Press the dough evenly into the bottom of the prepared pan. Chill in the refrigerator for 15 minutes, then bake for 15–20 minutes until lightly golden. Allow to cool completely.

In a large bowl, whisk together eggs, granulated sugar, all-purpose flour, key lime juice, and zest until smooth.

Pour the custard over the cooled crust and bake for 25–30 minutes until just set with a slight jiggle in the center. Cool completely in the pan.

Refrigerate the bars until cold, dust with powdered sugar, slice into 16 bars, and serve.