Finely crush 30 of the Oreos, using a food processor if possible.

Combine Oreo crumbs with 1/2 c. softened butter and press into 9x9 inch pan to form crust.

Chill crust until firm.

In a medium mixing bowl, combine powdered sugar, 1/2 c. softened butter, milk, and vanilla.

Mix in medium-high speed 2-3 minutes.

Crush remaining 10 Oreos, leaving some larger pieces, and stir into powdered sugar mixture.

Spread mixture over top of crust and keep refrigerated until ready to serve.