Step 1

Preheat oven to 350°F. Grease a 13- x 9-inch baking pan with baking spray.

Step 2

Beat cake mix, eggs, oil, lemon and lime zests, and 1 cup of the buttermilk in a large bowl with an electric mixer on medium speed 2 minutes. Pour into prepared pan.

Step 3

Bake in preheated oven until a wooden pick inserted in center comes out clean, 25 to 28 minutes.

Step 4

Meanwhile, whisk together sweetened condensed milk, lemon and lime juices, and remaining ⅓ cup buttermilk until smooth. While the cake is still warm and using the round handle of a wooden spoon, poke holes all over top of cake, about 1 ½ inches apart. Pour condensed milk mixture over cake, spreading evenly over top with a spatula. Cool until room temperature, about 1 hour.

Step 5

Spread whipped topping over top. Refrigerate until chilled, about 1 hour. Garnish with lemon and lime slices, if desired. Cake can be stored, covered, without whipped topping, in the refrigerator up to 3 days. Spread with whipped topping just before serving.