Preheat oven to 350 F; set oven rack in top third of oven. Beat together butter, sugar, salt and baking powder in a medium mixing bowl, until well blended. Beat in lime juice, zest, and egg. Blend in the flour thoroughly (dough will be rather stiff). If using macadamia nuts, incorporate now. Roll the dough into 1-inch balls and place them on a greased or parchment lined baking sheet. Bake for 12 to 16 minutes. Remove from oven. Cool on the pan for a few minutes. While still slightly warm, roll in powdered sugar. Allow cookies to cool completely on wire rack, then coat with sugar again.