Preheat your oven to 375ºF (190ºC) and prepare a baking sheet with parchment paper.

Cream the softened butter and dark brown sugar together until light and fluffy.

Add the large egg and mix until fully incorporated, resulting in a glossy batter.

In a separate bowl, whisk together the all-purpose flour, cinnamon, salt, baking soda, and baking powder.

Gradually add the dry mixture to the wet ingredients, stirring gently until just combined.

Fold in the finely diced rhubarb until evenly distributed in the dough.

Scoop the dough onto the prepared baking sheet, leaving 2 inches of space between each scoop.

Bake for 12–14 minutes until the edges are golden brown; the centers should be slightly soft.

Cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.