My kids love to help mix this up with their hands (so they can eat the fudge off their fingers afterwards of course) -- or you can use a mixer with a dough hook. All ingredients should be at room temperature. The recipe can also be halved but this freezes beautifully so don't hesitate to make the whole batch.
Mix confectioners' sugar, powdered milk, processed cheese food, butter, cocoa powder, walnuts, and vanilla extract in a large bowl with a stand mixer using the dough hook attachment until mixture is blended. The mixture will seem dry at first; continue mixing until smooth.
Press cheese mixture into a large baking sheet. Refrigerate until set, about 1 hour.