Preheat oven to 200°F and line 2 baking sheets with parchment paper. Make there are two evenly spaced oven racks in the oven.
In a stand mixer or using an electric hand mixer, whip egg whites until foamy.
Once foamy, continue to whip, slowly adding in the sugar.
Continue whipping until the meringue reaches stiff peaks.
Fold in the peanut butter (mixed in) until there are streaks of peanut butter, but no giant chunks.
Transfer the meringue to a large piping bag fitted with the piping tip of your choice.
Pipe a circle around 2.5 inches (6 cm) in diameter, then pipe over the edges 2 times to form a cup shape. Leave 1 inch (2.5cm) between each cookie.
Transfer the peanut butter (piped in each cookie) into a small piping bag. Then pipe around 1/2 tsp peanut butter into each cookie. The center of the cookie should only be around half filled.
Place both baking sheets into the oven and bake for 1 hour and 15 minutes, rotating the baking sheets to the other oven rack after 40 minutes.
Turn the oven off and leave the cookies in the oven for 1 hour, or until fully cooled.