1. Beat butter, 1 1/2 cups sugar, and lime zest on medium speed with an electric mixer fitted with a paddle until lightly creamed. Add sour cream, and beat just until blended. Stir in egg yolks and lime juice, scraping down sides of bowl. Add salt, baking powder, and flour, and beat until blended. Chill dough 1 hour.
2. Preheat oven to 350°. Line 2 baking sheets with parchment paper. Place remaining 1/2 cup sugar in a small bowl. Shape dough into 1-inch balls. Roll in sugar, and place on baking sheets.
3. Bake at 350° for 20 to 25 minutes or until edges begin to brown; bottoms will look light golden. (For crispier cookies, bake a few additional minutes.) Remove to wire racks to cool.
*Remove eggs from refrigerator, and place in a bowl for 30 minutes.