01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan.
02 - In a large mixing bowl, combine chocolate cake mix, eggs, water, and vegetable oil. Blend with an electric mixer or hand whisk until smooth and homogenous.
03 - Pour batter into prepared pan. Bake for 28 to 32 minutes, or until a toothpick inserted in the center emerges clean.
04 - Let the cake cool in the pan for 10 to 15 minutes.
05 - With the handle of a wooden spoon, create holes spaced about 1 inch apart across the cake surface.
06 - In a medium bowl, whisk instant vanilla pudding mix, cold milk, peppermint extract, and food coloring, if using, until thickened, about 2 minutes.
07 - Immediately pour prepared pudding over the cake, using a spatula to spread and press the mixture into the holes evenly.
08 - Cover and refrigerate for at least 1 hour, or until pudding is fully set.
09 - Gently spread the whipped topping evenly over the chilled cake surface.
10 - Scatter crushed peppermint candies and, if desired, chocolate shavings or mini chocolate chips over the topping.
11 - Slice and serve the cake chilled.