Preheat your oven to 375°F. While it heats, finely chop the rhubarb into 1/4-inch pieces and set aside on paper towels to absorb any excess moisture—this prevents the cookies from becoming too wet. Measure out all remaining ingredients and have them ready before you start mixing, so the cooking process moves smoothly.

In a medium bowl, combine the softened butter and brown sugar, beating them together until the mixture is light and fluffy, about 2-3 minutes. This creaming process incorporates air into the dough, which helps the cookies rise and creates a tender crumb. Add the room-temperature egg and vanilla extract, beating until fully combined and the mixture looks smooth and pale.

In a separate bowl, whisk together the flour, cinnamon, salt, baking soda, and baking powder. This ensures the leavening agents are evenly distributed throughout the dough. Add the dry ingredient mixture to the wet mixture from Step 2, stirring until just combined—don't overmix, as this can lead to tough cookies. I always fold gently with a spatula rather than beat, which keeps the cookies tender and prevents developing too much gluten.

Gently fold the chopped rhubarb from Step 1 into the dough, stirring just until evenly distributed. The rhubarb will release juice during baking, creating pockets of tartness throughout the cookies. Be careful not to overmix at this stage, as you want to preserve the tender cookie texture.

Using a spoon or small cookie scoop, form the dough into 1-inch balls and place them about 2 inches apart on a greased or parchment-lined baking sheet. Bake in the preheated 375°F oven for 12-14 minutes, until the edges are lightly golden but the centers still look slightly underbaked. The cookies will continue cooking on the hot sheet, so resist the urge to bake them until they look fully set—this keeps them chewy in the center.

Let the baked cookies rest on the baking sheet for 2 minutes to set slightly, then transfer them to a wire cooling rack to cool completely. This brief resting period helps them firm up enough to move without falling apart, while cooling on a rack allows air to circulate underneath and prevents them from becoming soggy on the bottom.