Preheat your oven to 375ºF (190ºC) and line a baking sheet with parchment paper.

In a mixing bowl, combine softened butter and brown sugar, then beat until light and fluffy (about 3 minutes).

Add the large egg and mix until fully incorporated (about 1 minute).

In a separate bowl, whisk together flour, cinnamon, salt, baking soda, and baking powder.

Gradually add dry ingredients to wet ingredients, stirring gently until just combined.

Fold in the finely diced rhubarb, ensuring even distribution.

Scoop dough onto the prepared baking sheet, spacing about 2 inches apart, and lightly flatten each mound.

Bake for 12–14 minutes, until edges are golden brown and centers remain soft.

Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack.