Preheat oven to 350°F. Line a 9x13” pan with foil and grease with butter or spray with cooking spray.
Cut the butter into 1-inch slices and place in a small saucepan. Melt over low heat and add the chocolate chips, stirring until the chocolate is melted and smooth. Turn off the burner but leave the pan on it (to stay warm).
Whisk the eggs in a large bowl. Add the sugar and salt and whisk until incorporated. Gradually whisk the chocolate mixture into the egg mixture. Whisk in the vanilla.
Place the flour and cocoa in sifter or a wire mesh strainer. Sift directly into the batter and stir with a spatula. Pour batter into prepared pan and spread until even. Place the 15 large Oreos evenly on top and press down lightly to cover with the batter.
Bake for 28-34 minutes until a toothpick inserted in the center comes out clean. Cool to room temperature on the counter, then cover and let sit overnight (you can also refrigerate it overnight).
Make the frosting: beat shortening and butter in a stand mixer with the paddle attached (or a hand mixer). Mix in marshmallow creme and vanilla. Slowly mix in powdered sugar. Spread frosting evenly over top of the brownies. Top evenly with mini Oreos.
Make the topping: melt the butter and corn syrup in a small saucepan. Turn off the heat and stir in the chocolate chips, then stir in the water to thin. Drizzle over the top of the frosting. Chill the frosted brownies for at least 1 hour until the frosting and topping sets. Remove from pan and slice into bars.