Heat oven to 350° F. Coat a 13 x 9-inch baking pan with nonstick cooking spray. Line bottom with wax paper. Coat paper.
In a large bowl, whisk together flour, baking powder and salt. Combine milk and extracts. In a medium-size bowl, beat egg whites until frothy and then beat in 1/4 cup of the sugar until stiff peaks form.
In a second large bowl beat remaining 11/4 cups sugar with butter on low. Beat in flour mixture, alternating with milk mixture. Beat 1 minute. Fold in beaten egg white mixture in 2 batches.
Spoon batter into prepared pan and spread evenly. Bake at 350° F for 30 minutes, until a toothpick inserted in center comes out clean. Cool in pan for 15 minutes. Turn out onto wire rack and cool completely.
In a large bowl, beat together butter, confectioners' sugar, lemon juice, milk and extracts until smooth.
Place toothpicks around the edge of cake, halfway up sides. Using picks as a guide, slice cake in half horizontally. Using two large spatulas, lift off top layer and place on a cutting board. Spread lemon curd on cut side of bottom layer. Gently replace top layer.
Reserve 1 1/2 cups of the frosting for piping a border and message. Spread cake with remaining frosting. Press coconut onto sides. Transfer 1 cup of the reserved frosting to a piping bag fitted with #21 star tip. Pipe shell pattern on top edge of cake. Tint remaining 1/2 cup frosting with food coloring, if desired. Transfer to bag fitted with writing tip and inscribe.
