Preheat oven to 350°F. Spray Fancy Fill pans with vegetable pan spray.
Prepare batter according to package directions. Divide batter evenly between prepared pans. Bake 20-25 minutes or until toothpick inserted in center comes out clean. Cool in pan on wire rack 10 minutes; remove from pan and cool layers completely.
Meanwhile, in small bowl combine eggs, egg yolk and sugar; mix until smooth. In small saucepan, bring lime juice to a boil; stir into egg mixture. Return to saucepan and heat, whisking constantly 3 minutes or until mixture is pale yellow and coats the back of a spoon. Remove from heat; add butter, vanilla and salt. Cool in the refrigerator at least 1 hour. Once cooled, fold whipped topping into mixture.
Fill tunnels of cooled cakes with lime cream mixture. Assemble cake by inverting top layer onto bottom layer. Ice cake with buttercream; press shredded coconut onto sides and top of cake. Refrigerate until ready to serve.
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