Preheat oven to 350 degrees F (175 degrees C). Grease cookie sheets.

In a large bowl, cream butter, sugar, egg, and egg yolk until smooth. Stir in lime juice and lime zest. Combine the flour, baking powder, and salt; blend into the creamed mixture. Form dough into ½ inch balls, and arrange on the prepared cookie sheet.

Bake 8 to 10 minutes in the preheated oven, or until lightly browned. Cool on wire racks. Sift confectioners' sugar over cookies while still warm.