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 whisk according to package directions until smooth.
03 - Pour the batter into the prepared pan and bake for 28 to 32 minutes, until a toothpick inserted in the center comes out clean.
04 - Let the cake cool in the pan for 10 to 15 minutes.
05 - Using the handle of a wooden spoon, poke holes over the entire surface of the cake, spacing about 1 inch apart.
06 - In a medium bowl, whisk instant vanilla pudding mix with cold milk, peppermint extract, and food coloring (if using) until thickened, about 2 minutes.
07 - Immediately pour and gently spread the pudding over the cooled cake, using a spatula to press the filling into the holes.
08 - Cover and refrigerate for at least 1 hour, or until the filling is set.
09 - Evenly spread thawed whipped topping over the chilled cake.
10 - Sprinkle with crushed peppermint candies and chocolate shavings or mini chocolate chips, if desired. Slice and serve chilled.