Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.
Combine flour, sugar, cocoa, baking powder, baking soda, and salt in a large bowl.
In another bowl, combine eggs, milk, vegetable oil, and vanilla extract.
Gradually add wet ingredients to the dry mixture and blend until smooth.
Pour batter into prepared pan and bake for 30 minutes or until a toothpick inserted into the center comes out clean.
Cool completely after baking.
Once cooled, use a fork or wooden skewer to poke holes evenly across the top of the cake.
Prepare instant pudding with cold milk according to package instructions.
Pour pudding evenly over the poked holes, allowing it to seep into the cake.
Chill in the refrigerator.
Spread Cool Whip or whipped topping evenly over the chilled pudding layer.
Tint a portion green with food coloring if you wish for a “melting witch” look.
Decorate with witch hat cookies and sprinkles.
Slice and serve chilled.