As the voucher says, FREE Cheeseburger with ANY purchase. So, buy a 30 cent soft-serve cone and get the burger free. Potentially, $1.40 gets you a McFlurry and 3 cheeseburgers.
EDIT: Just bought a soft-serve at McDonald's Bell St Preston. Handed a voucher over and the girl in the drive-thru didn't bat an eye lid. Drove off with a cone & burger for 30 cents. Do they even check these vouchers? Maybe others can try different stores as well with success.