Preheat your oven to 350°F (175°C).

In a large bowl, cream together the softened butter and brown sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

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

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

Fold in the finely chopped rhubarb.

Drop rounded tablespoons of dough onto a parchment-lined baking sheet, spacing them about 2 inches apart.

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

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