1. Lightly grease a swiss roll tin 28cm x 18cm/ 11″ x 7″ or line tin with greaseproof paper.
2. Put the brown sugar and syrup into a saucepan over a medium heat until just boiling, then remove from the heat.
3. Add the peanut butter and vanilla essence and stir until smooth.
4. Meanwhile in a large bowl, mix together the oats, sultanas and rice crispies.
5. Pour the saucepan contents over the cereals and mix well.
6. Press into the prepared tin using wet hands. Allow to cool, then cut into squares.
Smooth peanut butter can be used instead of crunchy.
Sultanas may be replaced with chopped cherries.
Wrap in greaseproof paper when cold to keep fresh and crunchy.