Creating the Oreo Dump Cake is as simple as following these easy steps:

1. Preheat your oven to 350°F (175°C). Prepare a 9×13-inch baking dish by greasing it lightly.
2. Crush Oreo Cookies: Place the Oreo cookies in a zip-top bag and crush them into coarse pieces. Optionally, leave some larger pieces for added texture.
3. Combine Ingredients: In a large bowl, mix the chocolate cake mix, melted butter, egg, water, milk, and vanilla extract. Stir the ingredients together until just combined; do not over-mix.
4. Layer Oreos: Evenly distribute half of the crushed Oreo cookies in the bottom of the greased baking dish.
5. Add Cake Mixture: Pour the chocolate cake batter over the layer of Oreos, spreading it evenly.
6. Add Toppings: Sprinkle the mini marshmallows, if using, and the remaining crushed Oreos on top of the batter, ensuring even coverage.
7. Bake: Place the baking dish in the preheated oven and bake for 45-50 minutes, or until a toothpick inserted in the center comes out clean.
8. Cool: Once baked, remove the cake from the oven and allow it to cool for 10-15 minutes before serving.

These straightforward steps will guide you in making this incredible Oreo Dump Cake effortlessly!