In a small bowl, use a fork or electric beater to mix the peanut butter, powdered sugar, and vanilla together. The mixture should resemble a soft clay. You want to be able to mold it and have it stay in place without melting. To firm up the mixture, add a tablespoon of powdered sugar at a time until the perfect consistency is reached. To soften the mixture, add a half-tablespoon of peanut butter until the perfect consistency is reached.