01 - Preheat the oven to 350°F. Generously grease a 9x13-inch baking pan.
02 - Combine chocolate cake mix, eggs, water, and vegetable oil in a large bowl. Mix thoroughly with an electric mixer or hand whisk according to box directions, until batter is smooth. Pour into the prepared baking pan.
03 - Bake for 28 to 32 minutes, or until a toothpick inserted in the center emerges clean. Remove from the oven and let cool for 10 to 15 minutes.
04 - Using the handle of a wooden spoon, poke holes evenly across the surface of the cake, spacing about 1 inch apart.
05 - In a separate mixing bowl, whisk instant vanilla pudding mix, cold milk, peppermint extract, and food coloring (if using) for about 2 minutes, until the mixture thickens.
06 - Immediately and evenly pour the pudding mixture over the cake, gently spreading and pressing it into the holes with a spatula.
07 - Cover and refrigerate the cake for at least 1 hour or until the pudding filling is fully set.
08 - Evenly spread the thawed whipped topping over the chilled cake surface.
09 - Sprinkle crushed peppermint candies and, if desired, chocolate shavings or mini chocolate chips over the topping.
10 - Slice into portions and serve chilled.