Preheat your oven to 350°F (175°C). Grease a 9-inch square baking pan.
In a mixing bowl, combine the chopped rhubarb with 1/4 cup of granulated sugar. Set aside to let it release some juices.
In another bowl, cream together the softened butter and remaining 3/4 cup granulated sugar until light and fluffy. Add the egg and vanilla extract, mixing until well combined.
In a separate bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon. Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined.
Fold in the rhubarb mixture gently until evenly distributed.
Pour the batter into the prepared baking pan, spreading it evenly. In a small bowl, mix together the brown sugar and a pinch of cinnamon. Sprinkle this mixture over the top of the batter.
Bake for 40-45 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool in the pan for about 10 minutes before slicing.