I've made this recipe over 200 times (slight variations on the same fudge), and I've finally figured out how to make it smooth (not grainy) every time. If you've asked for the recipe, then here are the ingredients and the procedure.
2 sticks (one cup) real butter
4 cups sugar
2/3 cup buttermilk
1 jar (7 ounces) marshmallow creme
1 cup Jif natural creamy peanut butter
Grease a 9 inch by 13 inch pan to receive the finished product.
Place two sticks of butter in a 3 quart saucepan over low heat--just above simmer.
Turn the burner off and move the pan to a cool burner.
Sugar crystals on the wall of the saucepan are your enemy. If sugar gets on the wall of the pan, you will probably have grainy fudge. Carefully place the first cup of sugar in the center of the melted butter.
Add the other three cups of sugar. Make sure that the sugar forms a "mountain" in the center of the butter and does not touch the side of the pan.
Add the buttermilk, carefully pouring it around the edges of the sugar.
Place the candy thermometer in the mixture and turn the burner on LOW (2 out of 6 or less).
Walk away. Do NOT stir. Wait until the candy reaches 249 degrees F. This will take 35 to 50 minutes.
While you are waiting, prepare the marshmallow creme and peanut butter.
Place the marshmallow creme in a bowl. Place the peanut butter on top of the marshmallow creme.
The photos show what the fudge will look like as it warms toward 249 degrees. Note that the wall of the pan is clean--no sugar crystals.
When the fudge reaches 249 degrees, set the pan on a cool burner and remove the candy thermometer. Quickly add the marshmallow creme/peanut butter into the center of the pan.
Mix on high for five minutes--do NOT touch the side of the pan with the mixer blades (have you noticed a "stay away from the side of the pan theme?").
Using the large spoon (which you cleaned the sugar crystals from earlier), empty the fudge into the greased 9 inch by 13 inch pan.
Allow to cool for at least an hour before cutting.
This keeps very nicely in the refrigerator or freezer, but I don't know how long since it always disappears before it has a chance to age.