Dissolve salt in milk and extract: combine milk and vanilla in a small dish. Stir in salt for 20-30 seconds to dissolve.

Crush the Oreos: place the Oreos in a well-sealed zip-top bag. Use a rolling pin, wine bottle, or can to roll back and forth over the Oreos until they’re fairly well crushed (pea-sized pieces or smaller). You can alternatively chop the cookies up on a cutting board. Beware, if you have large pieces, it will make it harder to frost your cake/cupcakes.

Beat butter: beat butter with a stand or hand mixer for 1 minute on medium speed until smooth.

Add sugar in three batches: add a third of the sifted powdered sugar to the butter and mix briefly on low to incorporate, then beat on medium-high speed for 1 minute until light and fluffy. Repeat that process with the two remaining batches of powdered sugar until all sugar has been added. The frosting should be very light and fluffy, and have significantly increased in volume (see notes).

Add in milk: add in milk mixture (be sure to scrape in the salt from the bottom of the container), and beat on low just to roughly incorporate ingredients, then medium-high for 1 minute until liquid is fully incorporated and frosting is fluffy again.

Add Oreos: mix crushed Oreos in on medium speed until evenly distributed.

Frost or chill: frost cake or cupcakes immediately if desired, or store covered in the fridge (see notes regarding color change of chilled frosting).