01 - Preheat the oven to 350°F. Lightly grease a 9x13-inch baking pan.
02 - In a large bowl, combine the chocolate cake mix, eggs, water, and vegetable oil. Mix on medium speed using an electric mixer or hand whisk until the batter is smooth.
03 - Pour the batter into the prepared baking pan. Bake for 28 to 32 minutes, or until a toothpick inserted in the center comes out clean.
04 - Allow the cake to cool for 10 to 15 minutes. Using the handle of a wooden spoon, poke holes over the surface of the cake, about 1 inch apart.
05 - In a medium bowl, whisk together the instant vanilla pudding mix, cold milk, peppermint extract, and food coloring (if using) until beginning to thicken, about 2 minutes.
06 - Immediately pour the pudding over the cooled cake, spreading evenly and pressing the mixture into the holes using a spatula.
07 - Cover and refrigerate the cake for at least 1 hour, or until the pudding is set.
08 - Spread the thawed whipped topping evenly over the chilled cake.
09 - Sprinkle crushed peppermint candies and chocolate shavings or chips over the whipped topping as desired.
10 - Cut into portions and serve chilled.