Line a 13x9 inch baking dish with wax paper (or grease generously)
In a large saucepan combine the sugar and milks. Set the heat to medium and bring to a hard boil, stirring continuously.
Boil for 10 minutes, without stirring, until the fudge reaches 236 degrees or the soft ball stage (get a bowl and fill it with ice water. Drip a teaspoon of the mixture into the ice water and when it can be formed into a ball without losing its shape that's soft ball stage. It should be the consistency of play dough and lose shape once removed from the water. If soft ball stage hasn't been reached continue cooking, checking in 30 second intervals.
After the mixture has reached soft ball stage remove the saucepan from the heat and let it cool down for 20 minutes.
Beat in the vanilla, salt and peanut butter with an electric mixer (easier), pour in the bowl of your stand mixer with the paddle attachment (easiest) or beat with a wooden spoon (great workout) until the mixture begins to get stiff.This is important because if you don't beat long enough you'll end up with a syrupy mess. Delicious, but not fudge.
Immediately pour the fudge into the prepared dish and spread the jelly on top. With a knife, swirl the jelly into and around the fudge.
Allow to cool then turn out onto a cutting board. Cut the fudge into desired size pieces and enjoy with a glass of cold milk.