In a separate bowl, mix together flour, corn starch, icing sugar and cinnamon.
Now add margarine and the soy flour mixture.
Once everything is combined well, you should be left with a smooth lump of dough, which may still be fairly sticky.
Wrap the dough in cling film and leave it in the fridge for at least 1 hour.
Preheat the oven to 180°C.
The dough is now still slightly sticky, but you should be able to form little balls using your hands. Make sure you make pretty small ones (32 in total), as they will spread a little in the oven.
Place them on a tray and remember to keep the biscuits apart quite a bit.
Now use your finger or the end of a wooden spoon (dusted with flour) to create a hole in the middle of each biscuit. Fill a small amount of jam in the hole with the help of a teaspoon.
Bake the thumbprint cookies for 10 minutes at 180°C.
Once you’ve removed the cookies from the oven and left them to cool, you can dust them with icing sugar.