01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan thoroughly to prevent sticking.
02 - Combine chocolate cake mix, eggs, water, and oil in a large mixing bowl. Blend according to cake mix package directions until batter is smooth and well combined.
03 - Pour the cake batter into the prepared 9x13-inch pan. Bake for 28 to 32 minutes, or until a toothpick inserted in the center emerges clean.
04 - Let the cake cool in the pan for 10 to 15 minutes to allow for easier handling.
05 - Using the handle of a wooden spoon, poke holes uniformly across the cake, spacing each hole approximately 1 inch apart.
06 - In a medium bowl, whisk together instant vanilla pudding mix, cold milk, peppermint extract, and food coloring if using. Whisk for about 2 minutes until the mixture thickens.
07 - Immediately pour the pudding evenly over the cake, using a spatula to spread and press the mixture into the holes.
08 - Cover the dish and refrigerate for at least 1 hour until the pudding is set and the cake is well chilled.
09 - Evenly spread thawed whipped topping over the surface of the chilled cake.
10 - Sprinkle crushed peppermint candies and, if desired, chocolate shavings or mini chocolate chips over the top.
11 - Cut into slices and serve chilled for optimal texture and flavor.