Preheat oven to 350°. Spray the bottom only of a 13x9-inch baking dish with nonstick baking spray with flour.
Prepare and bake cake mix in prepared dish according to package directions. Let cake cool for
In a medium bowl, stir together condensed milk, 1 cup coconut milk, and 6 tablespoons lime juice until blended.
Using the handle of a wooden spoon, poke holes in warm cake, making sure handle does not touch bottom of pan. Pour coconut mixture over warm cake. Cover and refrigerate until chilled, approximately 4 hours.
In a large chilled bowl, combine cream, confectioners’ sugar, remaining coconut milk, and remaining 4 tablespoons lime juice. Beat at high speed with an electric mixer and chilled beaters just until thickened; spread over cake. Top with lime zest and coconut, if desired. Store, covered, in refrigerator for up to 2 days.