Pass a piece of paper to your right, with a number X written on it, any number. Each person adds to the number on the first go-through. Cross out the previous sum and write the new sum anywhere on the paper. On the next go-through, each person subtracts their number, or if they want dessert, subtracts an additional 1.
X may be negative.
You'll end up with the number of votes. Everyone reveals if <= -2
To hide the number of votes, just require that it sum to a negative, rather than summing to negative 1
To determine if a unanimous yes has been established after hiding the number of votes but seeing that there was at least one yes, pass the paper around a third time if the sum <=-2, and everyone adds back the their random excess so that the sum of their three numbers is -1. if = number of participants, it is unanimous.
EDIT: guess we can do things without looking? Much easier then, instead of summing and all that, just write down 3 numbers between -5 and 5 that sum to 0 if you want dessert and any negative number if you don't want dessert, without looking. Then when everyone's done sum them.