Combine cookie crumbs and 1/3 c. butter in a medium bowl. Press onto bottom of an 9-inch square pan. Refrigerate until firm.

Combine powdered sugar, 1/3 c. butter, key lime juice, and key lime zest in a small bowl. Beat at medium-high speed until light and smooth. Spread over crust.

Score bars to desired size (approx. 1 1/2 inch squares) and top each square with a key lime slice.

Cover bars and refrigerate until firm, about 2 hours. Store in refrigerator. Remove lime slice before eating.

(*Recipe adapted from The Cookie Cookbook: Citrus Cream Bars)