In a medium or large saucepan bring the sugar, milk, and salt to a boil over medium-high heat. Be sure to stir continuously until it reaches a boil. Then turn it down to medium heat stirring occasionally until it’s finished.
If you are using a thermometer, your fudge is ready when the temperature reaches 235 degrees, or you can use the old way, which is the one I prefer. The old way is called the ball test. To do the ball test, you’ll need a cup of ice water and a metal spoon. After your fudge has been boiling for about five minutes, drizzle a little fudge from the pan into the cup of ice water. If you are able to form the fudge into a ball, then it’s done.
Remove from heat and add peanut butter and vanilla. You’ll want to work fast and stir until completely mixed and pour into a 8 x 8 baking pan before it begins setting up.
Put spoonful's of jelly on the top of the peanut butter fudge in different areas and use a knife to swirl the jam around. You’ll need to do this before the peanut butter fudge starts setting up.
Allow to sit for about 2 hours or until cooled and harden.