Line an 8x8-inch baking pan with aluminum foil or parchment paper.
In a large microwave-safe bowl, combine the powdered sugar, cocoa powder, milk, and vanilla. Add the butter last so that it sits on top of the other ingredients. Do not cut up the butter and do not mix the ingredients.
Microwave on high for 2 1/2 minutes. Remove the bowl from the microwave and mix until most of the sugar lumps dissolve. This takes about a minute of vigorous mixing. Stir in the marshmallows.
Return the bowl to the microwave and cook on high until the marshmallows melt, 20 to 30 seconds. Stir well again. Add any additional mix-ins if you like, like nuts or shredded coconut. Pour the fudge mixture into the prepared pan and smooth the top.
Cool in the refrigerator for at least 2 hours. Remove the parchment paper or foil from the pan. Cut the fudge into bite-size squares and enjoy.
This fudge is best stored, tightly wrapped, on the counter for 1 week.
If you want to keep it for longer, store it in the refrigerator for 2 to 3 weeks. It may dry out slightly in the fridge, so keep it tightly wrapped.