Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a mixing bowl, whisk together the flour, baking soda, and salt until well combined.

In another bowl, cream the softened butter and brown sugar until light and fluffy.

Add the egg and vanilla extract to the butter mixture and mix until combined.

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

Fold in the chopped rhubarb gently.

Drop tablespoon-sized balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.

Bake in the preheated oven for 10-12 minutes or until the edges are golden brown.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.