Preheat oven to 375°F and line a baking sheet with parchment paper.
In a large bowl, beat brown sugar, butter, vanilla extract, and egg with an electric mixer.
Add flour, cinnamon, salt, baking soda, and baking powder. Mix until dough forms.
Fold in finely diced rhubarb and beat again briefly.
Scoop 1-inch dough balls onto the lined baking sheet.
Bake for 12-14 minutes, until the edges are set and centers look slightly soft.
Cool on the tray for 2 minutes, then transfer cookies to a wire rack.
