Preheat the oven according to the chocolate cake mix package directions. Lightly grease a 9 x 13-inch baking pan.
Prepare the chocolate cake batter according to the package instructions. If desired, gently fold in the chocolate chips.
Pour the batter into the prepared pan and bake according to the package directions, or until a toothpick inserted in the center comes out with a few moist crumbs
Place the cake on a cooling rack and let it cool for about 15 to 20 minutes.
While the cake cools, whisk the instant chocolate pudding mix with cold milk until it begins to thicken. Stir in the peppermint extract and vanilla extract. Add green food coloring if desired.
Using the rounded handle of a wooden spoon, poke evenly spaced holes across the surface of the cake. Make the holes about 1 to 1 1/2 inches apart.
Spoon the mint chocolate pudding mixture over the cake, spreading it evenly so the filling can settle into the holes.
Cover the cake and refrigerate for at least 1 to 2 hours.
Stir the peppermint extract into the thawed whipped topping. Add powdered sugar if desired.
Spread the mint whipped topping evenly over the chilled cake.
Sprinkle chocolate shavings, chocolate chips, or crushed mint chocolate candies over the top.
Chill the finished cake for another 30 minutes if desired, then slice and serve cold.
