In a mixing bowl, cream butter and sugar until light and fluffy. Beat in egg, then add lime juice and lime oil, beating well. Add flour, baking powder and salt and mix only until combined. Cover and refrigerate dough about 1 hour. Using 1 level teaspoon of dough for each cookie, roll dough into balls in the palms of your hands. Lightly flatten the balls into round disks about 1 1/4 inches in diameter. Place cookies on lightly greased cookie sheets about 1 inch apart. Bake at 350° for 10 to 12 minutes, or until the cookies just begin to brown on the bottom. Meanwhile, put confectioners' sugar into a shallow dish, such as a pie pan. Let cookies stand 1 minute after removing them from the oven. Loosen with a spatula and place the warm cookies in the dish with confectioners' sugar, turning to coat both sides. Place cookies on wire racks to cool completely. Store in an airtight container. Makes about 80 cookies.