Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper or a silicone baking mat. This ensures the cookies bake evenly and release easily after baking.
Step 2: Combine Dry Ingredients
In a medium bowl, whisk together the flour, baking soda, cinnamon, and salt until evenly combined. This helps distribute the leavening and spices for consistent flavor and texture.
Step 3: Cream the Butter and Sugars
In a large mixing bowl, use an electric mixer to beat the butter, brown sugar, and granulated sugar together until light and fluffy — about 2–3 minutes. This step incorporates air, giving the cookies a soft, chewy texture.
Add the egg and vanilla extract, then beat again until the mixture is smooth and creamy.
Step 4: Combine Wet and Dry Mixtures
Gradually add the dry ingredients to the wet mixture, mixing on low speed or by hand until just combined. Avoid overmixing to keep the cookies tender.
Fold in the oats and finely chopped rhubarb until evenly distributed throughout the dough. The rhubarb will add small bursts of tart flavor in every bite.
Step 5: Shape and Bake
Using a spoon or cookie scoop, drop heaping tablespoons of dough onto the prepared baking sheet, spacing them about 2 inches apart to allow for spreading.
Bake in the preheated oven for 10–12 minutes, or until the edges are lightly golden and the centers appear set but still soft. The cookies will continue to firm up as they cool.
