Add the Oreo cookies to a large zip-top bag and crush them with a rolling pin until you have fine crumbs with a few small chunks. Alternatively, pulse them in a food processor, stopping before they turn into a paste to maintain a "dirt" texture.
1 family-size package Oreo cookies
Set aside about 1 cup of the Oreo crumbs for the final topping. If using butter, mix the remaining crumbs with the melted butter and press gently into the bottom of a 9×13-inch baking dish to form a light base. Do not press too firmly; this is not a firm cheesecake-style crust.
1 family-size package Oreo cookies, 2 tablespoons unsalted butter
In a large bowl, whisk the instant chocolate pudding mix with 3 1/2 cups cold milk for about 2 minutes, until it starts to thicken. Let it sit for 5 minutes to finish setting. The pudding should be thick yet spreadable when ready to layer.
2 packages instant chocolate pudding mix, 3 1/2 cups cold milk
In another large bowl, use a hand mixer or stand mixer to beat the softened cream cheese until very smooth and fluffy, about 2–3 minutes, scraping down the sides as needed. Beat in the powdered sugar and vanilla extract until smooth and fully combined.
8 oz cream cheese, 1 cup powdered sugar, 1 teaspoon pure vanilla extract
Slowly mix 1 cup cold milk into the cream cheese mixture. It will look thin at first. Gently fold in the thawed whipped topping with a spatula until the mixture is silky, thick, and well combined, taking care not to deflate it too much.
1 cup cold milk, 1 tub whipped topping (such as Cool Whip)
Spread about half of the cream cheese and whipped topping mixture evenly over the Oreo crumb base in the baking dish. Use an offset spatula or the back of a spoon to smooth it into an even layer; it does not need to be perfect.
Gently spoon the chocolate pudding over the cream layer in small dollops across the surface, then carefully spread it into an even layer. Adding the pudding in spoonfuls helps prevent the layers from mixing together.
Spread the remaining cream cheese and whipped topping mixture gently over the chocolate pudding layer. Sprinkle the reserved Oreo crumbs evenly over the top to create the classic "dirt" topping. Add extra crushed Oreos if you prefer a thicker crumb layer.
1 family-size package Oreo cookies, Additional crushed Oreos
Cover the baking dish tightly with plastic wrap or foil and refrigerate for at least 4 hours, or overnight if possible. Chilling allows the layers to thicken, the flavors to meld, and the crumbs to soften slightly.
Just before serving, decorate the top with gummy worms, chocolate shavings or mini chocolate chips, and/or additional whipped cream if desired. Cut into squares or scoop with a large spoon and serve chilled.
Gummy worms, Chocolate shavings or mini chocolate chips, Whipped cream or extra whipped topping