In a large bowl, combine powdered sugar and cocoa powder and stir until they seem reasonably well mixed. If the mixture seems like there are small clumps, you may sift it after its been measured.
Pour milk over sugar mixture but do no mix. Place cubes of butter on top. Microwave on high power 3-4 minutes until the butter has almost completely melted.
Immediately remove mixture from microwave and stir well until very well combined. You need to work fast because you want to spread the mixture while its still hot, but you want every spec of sugar to be well incorporated.
Once chocolate mixture is thoroughly mixed, stir in walnuts and vanilla.
Immediately pour warm fudge into prepared dish and spread to desired thickness. I only used about 60% of the surface of my baking sheet and made a 10x10 inch square with my fudge. It will only spread as far as you want, so make it as thick as you'd like the individual pieces to be.
Allow to fully set at room temperature. Our house is always freezing, so it didn't take much time. If you live in a warm house, it may take several hours or even a bit of refrigeration.
Lift foil out of dish and transfer fudge to a cutting board. Use a sharp knife to cut squares.