Mix the dry ingredients in a large bowl.

Add melted butter, vinegar, and water.

Mix until just combined. This batter will be thin.

Pour into a 9x13 pan that has been sprayed with cooking spray.

Bake in a preheated 325 degree oven for 35 to 45 minutes. Cool completely.

Beat the butter and marshmallow cream in the bowl of an electric mixer fitted with the paddle attachment, for 2 minutes. Reduce speed to low, and slowly add the powdered sugar, 1 cup at a time until well incorporated. Add the salt, vanilla, and heavy cream. Beat until combined.

Crush the Oreos with a rolling pin. I like to leave some big chunks, I like having big pieces in the frosting.

Fold into the frosting.

Spread frosting over cooled cake.