Place sugar and syrup together in large pan, on medium heat. You want to make sure this does not burn, so stir constantly. Mix sugar and corn syrup until blended, and bring mix to a boil for exactly one minute. (Do not over boil, as it will make them tough.)
Take the pan off the heat and quickly stir in the peanut butter, and mix. Then stir in the rice krispies, pour into an ungreased pan and press flat.
You can use a cookie sheet for thin ones, but I like mine thicker so I use a glass pyrex pan, about 9x12.
Let cool, cut into squares and serve.
Notes
Substitutions
This has not been tried, but Golden Syrup should be able to be substituted for the corn syrup if avoiding corn. Another nut, seed, or soy butter should be able to be used in place of the peanut butter.
Corn Substitutions: Corn is a common ingredient in products. Starch, modified food starch, dextrin and maltodextrin can be from corn. Consult with your physician to find out which corn derivatives you need to avoid. Many corn-free options are available in the US. Find out more about corn substitutions.
Gluten: Gluten is a protein found in specific grains (wheat, spelt, kamut, barley, rye). Other grains are naturally gluten-free but may have cross-contact with gluten-containing grains. Look for certified gluten-free products if you need to avoid gluten. Find out more about wheat and gluten substitutions.
Peanut Substitutions: There are many peanut-free products available in the US. Find out more about peanut substitutions.