01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan.
02 - Combine chocolate cake mix, eggs, water, and vegetable oil in a large bowl. Beat until smooth. Transfer batter to prepared pan and bake for 28 to 32 minutes, or until a toothpick inserted in the center comes out clean.
03 - Allow cake to cool for 10 to 15 minutes. Using the handle of a wooden spoon, poke holes across the surface, spacing them about 1 inch apart.
04 - Whisk instant vanilla pudding mix, cold milk, peppermint extract, and food coloring in a medium bowl until thickened, about 2 minutes.
05 - Pour pudding evenly over the cooled cake, spreading and working it into the holes with a spatula.
06 - Cover the pan and refrigerate for at least 1 hour until set.
07 - Spread thawed whipped topping evenly over cake. Sprinkle with crushed peppermint candies and chocolate shavings or chips, if desired.
08 - Cut into portions and serve chilled.