In a large microwavable bowl, stir together (with a whisk) one 500 g. box of icing sugar and 60 g. of unsweetened cocoa powder. Pour 1/4 cup (60 ml) whole milk over that (don't stir it), then chunk up 125 g. of butter over the top. Don't stir any of it.
Microwave it 3-4 minutes, until the butter is just melted. Remove immediately from microwave. Add 1 tsp (5 ml) vanilla extract and a large handful of chopped nuts (optional), then stir with the whisk until just smooth (a minute or so -- it may look a little bumpy, but that's normal). Immediately pour/spread this into a buttered baking tin (about 8" x 8"), and refrigerate for an hour or two, until it's set.
(Prepared the 3 underlined things ahead of time, because if you wait and do those AFTER you microwave the fudge, the fudge will set up in the meantime.)