If you chill your pan of blondies first, they will slice more neatly. Make sure to line your pan with non-stick foil so you can lift the uncut blondies from the pan and onto a cutting board. Then slice them into squares using a large, sharp knife. If your knife gets covered with blondie residue while slicing, make sure to clean it frequently. Use a ruler to mark off where you'll make each slice to provide some guidance.