Melt butter in large saucepan over low heat. Add cocoa and remove from heat. Stir in sugar and one egg at a time, mixing well after each. Add vanilla; then flour, salt and 2/3 c of crushed oreos.
Pour into greased 9×13 pan. Bake for 20-22 min. Do not overbake. Cool.
Place butter in mixing bowl and mix until creamy. Add cream cheese and mix until well-blended and creamy. Add 1/2 of powdered sugar and mix on low until well-incorporated. Add the rest of the powdered sugar and mix on low until well-incorporated. Msiix on medium-high until creamy. Add vanilla and mix until well incorporated. Add salt (if using unsalted butter) and milk. Mix well until a spreadable consistency.
Spread over brownies. Sprinkle with remaining crushed oreos.