Chop the rhubarb into small pieces and place into a small bowl. Add the extra 1/2 table spoon of sugar, the lemon zest (use more or less depending on how lemony you like your cookies), and lemon juice to the chopped rhubarb, mix well and allow to sit for a little while.
Cream butter and sugars, then add the egg and mix well.
Fold in flour and corn flour and then stir through rhubarb mixture.
Place table spoons (or bigger!) of dough on well greased trays and cook for 10-15 minutes.
These cookies will not brown, they stay very blonde. When you see them starting to go slightly brown around the edges, take them out, or they will be crumbly not soft and chewy.