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

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

In another bowl, cream the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the egg and vanilla extract to the butter mixture and mix until combined.

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

Fold in the chopped rhubarb gently.

Drop spoonfuls of the cookie dough onto the prepared baking sheet, spacing them about 2 inches apart.

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

Remove from the oven and let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.