Lightly grease a 20cm (8in) square tin. Put the condensed milk, butter, sugar and syrup in a large saucepan. Heat gently, stirring, until the sugar dissolves.
Bring to the boil, then simmer for 5 mins, stirring. Remove from the heat and allow to cool slightly. Stir in the rice snaps, then spoon into the tin and level the top.
When set, cut into 36 squares.