Mix sugar, cocoa and milk in a saucepan. Bring to a rapid boil and boil for exactly one minute. Pour over peanut butter and vanilla and nuts in a bowl and beat until thick - this only takes about a minute. Immediately pour onto a buttered platter and allow to cool. Cut into squares.
Tip: Have your peanut butter and vanilla already in a bowl before boiling the mixture as you need to pour it over the peanut butter immediately.