Melt 2/3 cup chocolate chips on the stove (low heat and stir constantly) or in the microwave. For details on how to melt chocolate in the microwave, see my chocolate covered strawberry tutorial: https://nashvillewife.com/chocolate-covered-strawberry-tutorial/
Place muffin liners in muffin tins, and fill 1/3 of the way with melted chocolate.
Place muffin pan in freezer for 20 minutes, or until chocolate is set.
Meanwhile, whip peanut butter and sugar with a fork until smooth.
Melt remaining 2/3 cup chocolate chips. Place a spoonful of peanut butter in the center of each muffin liner.
Top with melted chocolate so peanut butter is fully covered.
Freeze until chocolate is firm (at least 2-3 hours).
Makes approximately 6 large (regular sized muffin pans) peanut butter cups. For bite-sized peanut putter cups, use mini muffin pans.