Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a bowl, cream together butter and brown sugar until light and fluffy.
Add egg and vanilla extract; mix until smooth.
In another bowl, whisk together flour, cinnamon, salt, baking soda, and baking powder.
Gradually add dry ingredients to the wet mixture and mix until just combined.
Fold in the finely diced rhubarb gently using a spatula.
Scoop dough onto the prepared baking sheet, spacing them apart.
Bake for 12–15 minutes or until edges are lightly golden. Cool on a rack.