Step 1: Preheat your oven to 375°F (190°C). Line baking sheets with parchment paper or silicone mats.
Step 2: In a large bowl, using an electric mixer, cream together the softened butter, granulated sugar, and light brown sugar until the mixture is light and fluffy.
Step 3: Beat in the egg and vanilla extract until they are fully incorporated and the mixture is smooth.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, baking soda, and salt until well combined.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix the dough.
Step 6: Gently fold in the finely diced rhubarb until it is evenly distributed throughout the cookie dough.
Step 7: Drop rounded tablespoons of dough onto the prepared baking sheets, leaving about 2 inches of space between each cookie.
Step 8: Bake for 10-12 minutes, or until the edges are lightly golden and the centers appear set. Baking time may vary depending on your oven.
Step 9: Remove the cookies from the oven and let them cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. Enjoy!