In a medium bowl, whisk together the flour, rolled oats, baking soda, salt, and ground cinnamon (if using). Set aside.

In the bowl of a stand mixer with the paddle attachment (or using a hand mixer), cream together the softened butter, granulated sugar, and light brown sugar on medium speed until light and fluffy, about 2-3 minutes. Scrape down the sides of the bowl as needed.

Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix.

Gently fold in the finely diced fresh rhubarb with a rubber spatula until evenly distributed throughout the dough.

Cover the bowl with plastic wrap and chill the dough in the refrigerator for at least 30 minutes. This helps prevent the cookies from spreading too much and enhances flavor.

Preheat your oven to 375°F (190°C). Line two baking sheets with parchment paper.

Scoop rounded tablespoons of cookie dough onto the prepared baking sheets, leaving about 2 inches between each cookie. A cookie scoop works best for uniform size.

Bake for 10-12 minutes, or until the edges are lightly golden brown and the centers are set. The cookies may still look a little soft in the middle.

Remove the baking sheets from the oven and let the cookies cool on the sheets for 5 minutes before transferring them to a wire rack to cool completely.

While the cookies cool, prepare the glaze. In a small bowl, whisk together the powdered sugar, 1 tablespoon of milk, and 1/2 teaspoon of vanilla extract until smooth. If the glaze is too thick, add more milk 1/2 teaspoon at a time until desired drizzling consistency is reached.

Once the cookies are completely cool, drizzle the vanilla glaze over each cookie. Allow the glaze to set for about 15-20 minutes before serving.