Preheat oven to (350°F) (175°C).
Prepare two cookie sheets with parchment paper and set aside.
Mix together flour, baking powder and salt.
In a large bowl, cream together the butter and sugars. Add the egg and mix well.
Add dry ingredients to the creamed mixture and stir well. Dough may be a little dry at this point.
Add the rhubarb, lemon zest and lemon juice and mix well. The dough will be dry and crumbly but you can form into 1” balls easily and place on cookie sheets about 2” apart. Press down on the cookie to form a round disc.
Bake 15 -18 minutes. You will note a slight change in colour around the edges.
Cool on cookie sheet for 2 to 5 minutes then transfer to a cooling rack.