Preheat the oven to 350 degrees. Stir together the flour, salt, and baking powder in a bowl; set aside. In the bowl of a stand mixer, cream the butter and sugar together until smooth (about 3 minutes). Add the egg and mix, slowly adding the dry ingredients, until the dough comes together.
Roll tablespoons of dough into balls and place onto a baking sheet. Flatten the tops slightly with your hand before putting them in the oven. Bake for 12-15 minutes until they are brown around the edges. Set aside on a wire rack to cool.
While the cookies cool, make the curd by whisking the eggs, lime juice, and sugar in a saucepan over medium heat. Whisk in the butter a bit at a time. Whisk constantly for about 15 minutes until thickened. Strain through a sieve and leave to cool.
When both the cookies and curd are cooled, use a knife or spoon to spread the curd on the cookies. Top with either store-bought whipped topping, or whip sugar with heavy cream to make your own. Store unused cookies and curd separately, as the curd can soak into the cookies and make them lose their crunch if left too long.