In a medium size bowl, whisk together the peanut butter, confectioners sugar, butter, vanilla, and salt until smooth. Transfer the peanut butter mixture to the chocolate-coated tart pan and evenly spread with a flexible spatula.
Microwave the remaining chocolate and oil in the same microwave-safe bowl you previously used, and pour it over the peanut butter filling, smoothing the top with an offset spatula (or spatula).
Freeze until the sides of the pan easily pop off, about 30 minutes. The longer you wait, the easier they will pop out.
Once removed, use a small pairing knife to dislodge the base from the bottom of the cup. Let sit a room temperature for 5-10 minutes before cutting. Slice with a large chef's knife, dipping the knife in hot water and drying it in between cuts for the cleanest slices.
Keep the peanut butter cup, wrapped, in the refrigerator for up to a week.