Place the ½ cup (113 g) softened unsalted butter, ½ cup (96 g) vegetable shortening, and the entire 1 (7 oz / 198 g) jar of marshmallow fluff in a large mixing bowl. Using a stand mixer or hand mixer, beat on medium speed until the mixture is smooth and well combined, scraping down the sides of the bowl as needed.

With the mixer on low speed, add the 3 cups (339 g) powdered sugar to the bowl 1 cup at a time, mixing until each addition is fully incorporated before adding the next. Scrape down the bowl and beat again briefly to ensure there are no lumps.

Add the 1 teaspoon salt, 1 teaspoon vanilla extract, and 1 tablespoon heavy whipping cream. Beat on low to medium speed until the frosting is smooth.

If the frosting is too thick for your desired consistency, add up to 1 additional tablespoon of heavy whipping cream a little at a time, mixing briefly after each addition, until you reach the desired spreadable or pipeable texture.

Coarsely chop the 10 Oreos and fold them into the frosting by hand with a spatula, or mix on low speed for just a few seconds until they are evenly distributed. Avoid overmixing so the cookie pieces stay chunky.

Use the frosting to frost cupcakes, cake, or as desired.