1Heat oven to 350 degrees. Line a 13x9-in baking pan with foil,letting foil extend about 2 inches above pan at both ends. Coat foil with nonstick spray.
2Spread cake batter in prepared pan. Bake as box directs. Cool completely in pan on a wire rack. Holding foil ends. lift from pan. Trim rounded top (hump) off the cake with a long serrated knife until flat. Holding foil ends, return cake to pan.
3Pour lime juice into a small saucepan; sprinkle with the gelatin. Let stand 1 minute, then stir over low heat 3 minutes until gelatin completely dissolves. Remove from the heat.
4Beat cream cheese, sugar and lime peel in a large bowl with mixer on medium speed for 2 minutes until fluffy. Slowly beat in gelatin mixture. Beat 1 minute to blend. Scrape onto cake, then spread evenly to edges. Refrigerate 1 hour to set, then cover loosely and refrigerate 2 hours or overnight.
5To serve: Lift from pan. Cut into 42 bars (dip knife in hot water; wipe clean between cuts). To garnish: With citrus zester, cut peel off lime. Cut lime in quarters lengthwise and then crosswise into 11 slices. Top bars with lime and peel.