I adore the recipe for Nigellas Peanut Butter Squares (Domestic Goddess, p.223), and they are always a yummy treat that are perfect to bring to parties. However, I always find it a challenge to cut them. My best effort so far has been to use a warm knife to slice through them. However, this can be smudgy and messy. If I cut them with a room temperature knife, the chocolate starts to fracture and break. Would you have a solution to this problem?
With the Peanut Butter Squares, and with many items with a chocolate topping, it is useful to try and cut the bars before the chocolate has quite fully set, or alternatively let the squares come up to room temperature before cutting them, so that the chocolate softens slightly and is easier to cut. You can refrigerate the bars again after cutting. Use a sharp heavy bladed knife to get clean cuts. Also it is useful to wipe the knife clean between each cut so that the squares don't get smeared with chocolate and peanut butter.
If you are still having trouble then warm a sharp knife by dipping it in hot water and wiping it before each cut. Sometimes a serrated bread knife also works as you can saw slightly to cut through the crust then press down to complete the cut, though this is best for slightly squidgy brownies that have a crisp crust.