Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
Cream together butter and brown sugar in a large mixing bowl until light and fluffy.
Add the egg and vanilla extract and beat until fully combined.
Whisk together flour, baking soda, and salt in a separate bowl.
Gradually add dry ingredients to the wet ingredients, mixing until just combined.
Fold in chopped rhubarb, ensuring it's evenly distributed throughout the dough.
Scoop tablespoon-sized portions of dough onto the baking sheet, spacing them about 2 inches apart.
Bake for 10-12 minutes, or until the edges are golden brown and the center is set.
Cool on a wire rack for a few minutes before transferring the cookies to an airtight container.