1. Heat oven to 350 degrees F. In the bowl of a stand mixer fitted with the paddle attachment, beat butter on medium-high until creamy, about 3 minutes. Add sugar and beat another 3 minutes on medium-high, until mixture is fluffy. Add egg and vanilla; beat on medium until combined. Add flour, baking powder and salt; mix on low until just combined.
2. Form dough into 2 disks. Wrap in 2 separate pieces of plastic wrap. Refrigerate 30 minutes.
3. Sprinkle a large piece of parchment paper with flour. Roll out 1 disk until 1/8 thick. Using a 21/2-inch round cookie cutter, cut out 24 cookies. Place rounds on baking sheets about 1 inch apart. Repeat with second disk for a total of 48 cookies. Refrigerate dough for 10 minutes.
4. Bake cookies at 350 degrees F for 12 to 14 minutes or until lightly golden. Allow cookies to cool on baking sheet for 1 minute, then transfer to a wire rack to cool completely. (Baking might take 2 batches.)
5. On 24 of the cookies, dollop 2 tsp of the marshmallow creme. On remaining 24 cookies, dollop 2 tsp of the Jif spread. Make 24 sandwich cookies using 1 marshmallow cookie and 1 Jif cookie. Freeze cookie sandwiches 30 minutes.
6. Meanwhile, fill a small pot with 1 inch of water. Bring to a low simmer. Combine chocolate and oil in a metal bowl large enough to rest on top of pot. Stir until chocolate is melted.
7. Rest a frozen cookie sandwich on a 2-pronged barbecue fork and dip it into chocolate; flip to coat well on both sides. Remove, again resting cookie sandwich on fork, tapping utensil lightly on side of bowl to remove excess chocolate. Repeat with each sandwich. Transfer cookies to parchment paper. Let stand 1 hour or until chocolate sets.