Using a pastry cutter or in a food processor blend together the butter, sugar, salt and flours.
Press the shortbread dough evenly into the bottom of a 9x13 inch baking pan lined with parchment paper, allowing the paper to hang over for easy lifting later for cutting. Bake for 25 minutes until golden brown.
Allow the shortbread crust to cool for about 10 minutes.
In the food processor add the egg yolks, condensed milk, rum extract, lime juice, rum extract and mint leaves. Pulse until combined and pour over the baked shortbread base.
Bake for another 20 minutes or until the lime custard appears to be set. Cool completely, cut into squares and top with powdered sugar.