Preheat your oven to 375ºF (190ºC). Prepare your baking sheets by greasing them or lining them with parchment paper.

In a large mixing bowl, cream together softened butter and brown sugar until smooth and light in color, about 2-3 minutes. Add in the egg and mix until fluffy.

In a separate bowl, whisk together the all-purpose flour, cinnamon, salt, baking soda, and baking powder, then set aside.

Gradually incorporate the dry mixture into the wet ingredients, mixing gently until a soft dough forms. Adjust dough consistency if necessary.

Fold in the finely diced rhubarb gently into the cookie dough.

Using a cookie scoop or tablespoon, portion out the dough onto prepared baking sheets about 2 inches apart.

Bake in the preheated oven for 12-14 minutes until the edges are golden-brown and centers are slightly soft.

Remove from the oven and let the cookies cool on the sheet for 5 minutes before transferring to a wire rack.