In a large mixing bowl, using a stand or a hand mixer, beat the butter, sugar, and egg until smooth and creamy.
Add flour, ground cinnamon, baking soda, baking powder, and salt, and slowly beat again until just combined.
Fold in the diced rhubarb, then using a small 1-inch cookie scoop, scoop out balls of dough and place them onto the prepared cookie sheet.
Bake at 375°F/190°C for 12-14 minutes, or until set and lightly brown at the bottom.