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

In a large mixing bowl, cream together softened unsalted butter and brown sugar until light and fluffy, about 2-3 minutes.

Add the egg and vanilla extract, mixing on low until well combined.

In a separate bowl, whisk together flour, baking soda, and salt. Gradually mix into wet ingredients until just combined.

Gently fold in diced fresh rhubarb, ensuring even distribution.

Drop rounded tablespoons of dough onto the prepared baking sheet, spaced about 2 inches apart.

Bake for 10 to 12 minutes or until the edges are lightly golden and centers are soft.

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