From the myths and misconceptions feature on the SW website
2. Are couscous cake, Smash pizzas and lasagne-sheet 'Doritos' Free?
You might be disappointed to hear that they're not. Because changing the use of a food can make it easier to over-eat, to protect your weight loss they become Synned.
When foods like pasta, rice and grains are used in their original form they're low in energy density (calories), bulky, appetite-satisfying foods. Pasta, couscous and rice are usually the main ingredient in a meal packed with other Free Foods - giving it sufficient bulk to be satisfying as a meal in itself. However, when ground down, used as flour (like in couscous cake), used as a sauce thickener (like Smash), seasoned crisps (like Doritos) or puréed (like smoothies) they're often used as additions to a main meal... and on top of your day's Syns. It is a very effective way of adding a lot of extra calories in a non-bulky way, and as such would then be counted as Syns
Dry powdered mash is 4.5 syns per 25g on the red plan so that is the rate that should be used to work out the syns. The recipe at the top of this thread uses 120g so that works out at 21.5 syns