01 - Preheat the oven to 350°F. Grease a 9x13-inch baking pan to prevent sticking.
02 - Combine chocolate cake mix, eggs, water, and vegetable oil in a large mixing bowl. Mix according to package instructions until batter is smooth. Transfer batter to prepared pan and bake for 28 to 32 minutes, until a toothpick inserted in the center emerges clean.
03 - Allow cake to cool for 10 to 15 minutes. Using the handle of a wooden spoon, poke holes across the entire surface, spacing holes approximately 1 inch apart.
04 - In a medium bowl, vigorously whisk instant vanilla pudding mix, cold milk, peppermint extract, and food coloring if using, until thickened (about 2 minutes).
05 - Evenly pour pudding mixture over the cooled cake, spreading gently with a spatula to ensure pudding seeps into all holes.
06 - Cover and refrigerate for at least 1 hour to set the pudding filling.
07 - After chilling, spread thawed whipped topping evenly over the surface of the cake.
08 - Sprinkle crushed peppermint candies over the whipped topping and scatter chocolate shavings or mini chocolate chips if desired.
09 - Cut into portions and serve chilled.