Lightly butter an 8-inch-square baking pan and set aside. Combine corn syrup, both sugars, milk, baking soda, and salt in a medium-heavy saucepan. Bring to a boil over medium-high heat, stirring occasionally. Insert a candy thermometer. Cook, stirring frequently, until temperature registers 236 degrees (just below soft-ball stage); don't overcook.
Immediately remove saucepan from heat, and add chilled butter, then peanut butter and vanilla, stirring until smooth. Pour the mixture into prepared pan. Set aside in a cool, dry place, and let fudge set.
Before the fudge cools completely, cut into desired shapes. Using a small melon baller, scoop out a small hole in the center of each fudge shape. Set shapes aside to cool completely. Fill a resealable plastic bag with jelly; close, and cut off a corner. Squeeze the jelly into holes. Store fudge in an airtight container up to one week.