Melt dark chocolate candy melts in the microwave, in 30-second intervals or until completely melted.
Pour a quarter of the chocolate in the tart pan, completely coating the bottom and the sides. Place in freezer for 5-10 minutes until set. Repeat process 1 more time.
In a large bowl, cream together peanut butter and powdered sugar until combined. Pour mixture into tart pan, spreading it evenly across. Place pan in freezer for 10-15 minutes to set. Pour remaining chocolate on top of peanut butter mixture, and place in freezer for 30 minutes to completely harden.
Melt white candy melts in the microwave, in 30-second intervals, and then pour into a piping bag. Snip the tip, and pipe the symbol π on top of the peanut butter cup.