Preheat the oven to 375 degrees.

Cream the softened butter and sugar together for 2 minutes, scraping sides of bowl as needed.

Add in the egg yolk and vanilla extract, and beat for an additional minute.

Add the flour and salt, and let beat until the dough comes together and leaves the sides of the bowl.

Use your hands to roll the dough into 1 inch balls and place on a baking sheet that's covered with a Silpat or parchment paper.

Once all of the balls are rolled out, use your thumb to create an indention. Don't press through to the bottom of the dough. You can make the indention deeper after baking.

Bake for 9 minutes.

Meanwhile, heat up the peanut butter in a small ramekin in 10 second blasts, two times, until the peanut butter is more liquidy.

Remove the cookies from then oven, and use the rounded end of a wooden spatula to press the indentions a bit deeper, again not going through to the bottom of the cookie.

Use a smaller spoon to carefully put a small drop of peanut butter in each cookie, and try not to fill the indention more than halfway with peanut butter.

Use another spoon to repeat the process with the jelly, making sure not to overfill the indention.

Bake for another 3 minutes, and let cook on a baking rack.