01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking pan.
02 - In a large bowl, blend together chocolate cake mix, eggs, water, and vegetable oil using an electric mixer or whisk until mixture is smooth.
03 - Pour batter into prepared pan and bake for 28 to 32 minutes, or until a toothpick inserted in the center emerges clean.
04 - Let cake cool on a rack for 10 to 15 minutes.
05 - Using the handle of a wooden spoon, create holes over the cake surface about 1 inch apart.
06 - In a medium bowl, whisk instant vanilla pudding mix, cold milk, peppermint extract, and food coloring (if using) for about 2 minutes until slightly thickened.
07 - Immediately pour pudding evenly over cake, gently spreading and pressing pudding into holes with a spatula.
08 - Cover cake and refrigerate for at least 1 hour until fully set.
09 - Spread thawed whipped topping evenly across chilled cake.
10 - Sprinkle crushed peppermint candies and chocolate shavings or mini chocolate chips over surface.
11 - Cut into portions and serve chilled.