I tried to make peanut butter fudge using this recipe (which I got off the internet): Bring 2 cups granulated sugar and 1/2 cup milk to a boil. Allow to boil for 2 1/2 minutes, stirring regularly, and then remove from heat. Add 1 cup peanut butter and 1 tsp vanilla extract and stir until smooth. However, when I added the vanilla extract and peanut butter, it turned into a crumbly, dry mess. It could not be poured into a pan so I tried scooping the crumbly mess into a pan. When it cooled, it was hard as a rock and ruined. What did I do wrong or was the recipe wrong? I prefer to make my fudge without marshmallow cream.
For fudge to work you need to get it to the right temperatures. It is difficult (but not impossible) to do this without a sugar thermometer. I would recommend getting one if you are new to candy making.
It sounds like you didn't let the temperature drop enough and stir to allow microcrystal formation so it precipitated sugar crystals once you added the peanut butter (it acts as a nucleation point for crystal formation).