Over low heat, melt the butter. Don't burn it. Integrate the cream. Slowly add the sugar in while stirring. This makes a syrup. Raise the heat until syrup boils. Reduce heat to medium-low so that your syrup is bubbling, but not burning. You are trying to reduce the syrup a bit.
There are several thoughts on how to determine when the syrup is done reducing. The classic one is called the "soft ball test," in which a small sample of the syrup is dropped into a glass of ice water. If the syrup congeals into structured globs of goo, the syrup is down. The more scientific approach is to boil the fudge for 3 minutes with the pot lidded. Reduce heat to a simmer. Heat until syrup is 234°F. I sort of eyeball it. Getting the syrup right is the key to the fudge's final structural integrity. Luckly you, the peanut butter covers up a multitude of incompetance.
Once the syrup is done, remove pot from heat and let it stand for a few minutes. This stuff is lava-hot. In the mixing bowl, gather the peanut butter and marshmellows (or Fluff). Sometimes a quick stint in the microwave makes peanut butter more amenable to moving.
Add the syrup to the mixing bowl. Mix until blended. Some people will like a less homogenized fudge than others. This is controlled by how much you mix the fudge at this point. I go for the full-on integration.
Pour contents of mixing bowl into the greased 8x8 pan. Butter works best, but Pan will do. Remove pan to a cool, dry location, like your gargage or window sill. Let fudge sit for 30 minutes until cool to the touch.
When solidified, dump fudge onto cutting board and cube into 1" blocks.
Test the batch with samples until satisfied.
Yield: about 2 pounds
Cook time: ~15 minutes
What's sad is that I had enough ingredients left over to make chocolate fudge too. That's basically the same recipe, but with semi-sweet chocolate chips in place of peanut butter. I used about 2 TBSP of corn syrup and I think that was too much. Use only 1 TBSP. I'd also suggest using only 1/4 C of marshmellow too. When I've got a good recipe for chocolate fudge, I'll post it here. I think I'll need to use cocoa power rather than solid chocolate. I made a mean chocolate syrup with cocoa power once...