Preheat the oven to 350°F (175°C).

In a mixing bowl, cream together the softened butter and granulated 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 mixture, mixing until just combined.

Fold in the diced rhubarb and brown sugar.

Drop spoonfuls of dough onto a baking sheet lined with parchment paper.

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

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