Preheat the oven to 350°F (175°C). In a mixing bowl, combine flour, sugar, melted butter, and salt. Mix until crumbly.

Press the mixture into the bottom of a greased 9x9 inch baking pan. Bake for 15 minutes or until lightly golden.

In another bowl, whisk together eggs, sweetened condensed milk, key lime juice, sugar, and lime zest until smooth.

Pour the filling over the baked crust and return to the oven. Bake for an additional 10 minutes.

Remove from the oven and let cool completely. Refrigerate for at least 2 hours before cutting into bars.

Serve chilled, optionally dusted with powdered sugar.