Preheat the oven to 350 degrees.

In a large bowl cream together the butter, brown sugar, and sugar. Add in the egg and mix well.

In a medium bowl combine the flour, baking powder, and salt.

Add the dry ingredients to the wet ingredients and mix well. Your dough may be a little dry at this point.

Stir in the rhubarb, lemon zest, and lemon juice and mix until well combined.

Drop the dough onto ungreased cookie sheets by heaping tablespoons. Place the cookies at least 2 inches apart.

Bake for 12-15 minutes or until the cookies are just starting to brown slightly.

Remove from the oven and allow to cool for 2 minutes before removing them to a wire rack to cool completely.