Combine corn syrup and butter in a 2 quart saucepan. Cook over low heat, stirring, until butter melts. Remove from heat and stir in peanut butter. Return to low heat and cook and stir until smooth.
Add powdered sugar and vanilla continuing to stir, with a wooden spoon, until smooth. Spread into a greased 8-inch square baking dish. Refrigerate at least 2 hours before cutting into 1-inch squares.
Notes
It is not recommended to use natural peanut butter for this recipe.