Preheat oven to 350°F.
Spray an 8 x 8 x 2-inch baking pan with nonstick cooking spray; set aside.
In small mixing bowl combine sugar and egg and, using an electric mixer, beat until light and fluffy; add peanut butter and vanilla, beat-ing until combined.
Spread mixture evenly in sprayed pan and bake until top is lightly browned, about 20 minutes.
Remove pan to wire rack and let cool for 5 minutes; invert pan onto rack to remove mixture.
Let cool completely.
In small saucepan melt chocolate over low heat, stirring frequently.
Spread melted chocolate over cooled mixture and let harden.
Cut into sixteen 2-inch squares; wrap squares individually in plastic wrap and store in refrigerator.