Preheat the oven to 375ºF and coat a large baking sheet or cookie sheet with butter or nonstick cooking spray.

In a large bowl, cream the brown sugar and butter together until light and fluffy. Add the egg and beat to incorporate.

Add the flour, cinnamon, salt, baking soda, and baking powder. Beat well until the dough is well combined.

Add the chopped rhubarb and beat to incorporate it.

Using a cookie scoop (1 inch), form balls of dough and place them on the prepared baking sheet.

Bake cookies in the preheated oven for 12 to 14 minutes or until golden brown and baked through. Transfer to a wire rack to cool completely.