Prepare cake mix with water, oil, and eggs (see recipe note) according to package directions. Use a spoon to fold 1/2 cup sweetened shredded coconut (not toasted), lemon zest, lemon juice, and poppy seeds into batter. Pour into a greased 13" x 9" baking pan. Bake at 350 degrees according to package directions for a 13" x 9" pan (approximately 29 to 32 minutes) or until a toothpick inserted in the center comes out clean. Cool completely.
My Betty Crocker cake mix called for the amount of water, oil and eggs shown in the ingredient list above. Check your cake mix package for ingredient amounts and cooking times. The cake shown in this post was prepared with 3 whole eggs. For a lighter white cake, use just the egg whites.
To toast coconut: Preheat oven to 325 degrees F. Place 1/3 cup coconut on parchment paper lined baking sheet and transfer to the oven. Bake for 4 minutes. Toss with a spatula and return to oven for an additional 3 to 4 minutes, or until browned to your liking.
Make-Ahead Instructions: Follow as written but after spreading pudding mixture on top of cooled cake, cover tightly with plastic wrap or foil and refrigerate overnight. Spread with whipped topping and garnish with toasted coconut and lemon slices just before serving.
Adapted from Taste of Home