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

Cream the softened unsalted butter and brown sugar together until light and fluffy, about 2-3 minutes.

Beat in one egg and vanilla extract until smoothly combined.

In another bowl, whisk together the flour, baking soda, and salt, then gradually mix into the wet ingredients.

Fold in the chopped rhubarb until evenly distributed.

Scoop portions of dough onto the prepared baking sheet, leaving space between each.

Bake for 10-12 minutes until edges are golden and centers are soft.

Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.