In a medium bowl, stir together peanut butter, confectioners' sugar, salt, and vanilla. Transfer to a piping bag fitted with a 3/8-inch tip.
Line a baking sheet with parchment and pipe peanut butter disks about 3/4- to 1-inch in diameter and 1/4-inch thick. If needed, lightly press flat with a dampened finger. Place in freezer to harden for about 30 minutes.
Place baking cups on a baking sheet.
Melt and temper half of the chocolate (see our post on Microwave Tempering), and partly fill cups.
While chocolate is still soft, press a peanut butter disk into the center.
Melt and temper remaining chocolate, and fill cups to cover the peanut butter. Transfer to the refrigerator to harden.