Preheat oven to 350°. Spray a 13×9-inch baking pan with cooking spray.
In the bowl of a stand mixer fitted with paddle attachment, beat butter and sugar at medium speed until fluffy, 3 to 4 minutes. Add egg yolks, one at a time, beating well after each addition. Add lemon zest and juice and extracts, beating to combine.
In a medium bowl, combine flour, baking powder, baking soda, and salt. Gradually add flour mixture to butter mixture alternately with coconut milk, beginning and ending with flour mixture, beating just until combined after each addition.
In a medium bowl, beat egg whites with a mixer at high speed until stiff peaks form. Gently fold egg whites into batter. Spread batter into prepared pan.
Bake until a wooden pick inserted in center comes out clean, 25 to 30 minutes. Let cool in pan. Remove cake from pan. Spread Lemon Coconut Frosting over cake. Garnish with coconut and lemon, if desired.
Lemon-Coconut Frosting
In the bowl of a stand mixer fitted with paddle attachment, beat butter until creamy. Beat in zest and coconut extract. Gradually add 1 cup confectioners’ sugar alternately with 1 tablespoon coconut milk, beating on low until smooth. Increase speed to medium and beat until fluffy, 1 to 2 minutes. Add remaining 1 tablespoon coconut milk to achieve a spreadable consistency, if needed. Use immediately.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/lemon-coconut-sheet-cake/