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

In a large mixing bowl, cream together softened unsalted butter and brown sugar until smooth and fluffy.

Incorporate the egg and vanilla extract, beating until fully combined.

In a separate bowl, whisk together all-purpose flour, baking soda, and salt.

Gradually mix the dry ingredients into the wet mixture until just combined.

Fold in the diced fresh rhubarb gently to maintain the soft dough.

Using a cookie scoop, drop rounded tablespoons of dough onto the baking sheets, spacing them 2 inches apart.

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

Remove from the oven and allow to cool on the baking sheet for 5 minutes before transferring to a wire rack.

Enjoy your Brown Sugar Rhubarb Cookies warm or store them for later!