Preheat oven to 325°F. Spray a 9x13-inch baking dish with cooking spray.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt.
Add the melted butter, vinegar, and water.
Mix until just combined. The batter will be thin.
Pour the batter into the prepared dish.
Bake for 35 to 45 minutes. Insert a toothpick in the center. If the toothpick comes out with a few moist crumbs, your cake is done. Allow the cake to cool completely, about 1 hour.
In the bowl of a stand mixer, use the paddle attachment to beat the butter and marshmallow cream. Do this for 2 minutes. Reduce the mixer speed to low, adding the powdered sugar 1 cup at a time until well blended. Then, add the salt, vanilla extract, and heavy cream. Mix until combined and fluffy.
Crush the Oreos with a rolling pin.
Fold crushed Oreos into the frosting.
Spread frosting over the cooled cake.