Combine the egg yolks, butter and caster sugar in a large mixing bowl and whisk using an electric hand-held whisk until thick and creamy. Add the sifted flour and salt and bring together using a wooden spoon until you have a dough. Cover the bowl and chill for at least 30 minutes.
Pre-heat the oven to 160° C.
Roll the dough out to roughly ½cm thickness, then arrange on lined, greased baking trays, spaced apart. Bake for 10-12 minutes until golden brown. Remove and allow to cool on the trays for 5-10 minutes before transferring to a wire rack to cool completely. When ready to serve, top with the diced apple, lime zest and mint leaves.