Fill a disposable pastry bag with remaining liquid peanut butter (if peanut butter has thickened, microwave it 10 seconds), and snip off end.
Pipe peanut butter in continuous diagonal zigzag lines down length of tart. Use a wooden skewer to give it lots of swirls.
Refrigerate tart until set, at least 1 hour and up to 8 hours. Cut into wedges, and serve.