Preheat the oven to 350°F (175°C).
Grease and flour a round cake pan.
Mix the chopped rhubarb with 1/2 cup of granulated sugar and set aside.
Cream together the softened butter and remaining sugar until light and fluffy.
Add eggs one at a time, beating well after each addition.
Stir in the vanilla extract.
Mix the flour, baking powder, baking soda, and salt in a separate bowl.
Gradually add the dry ingredients to the butter mixture, alternating with the rhubarb mixture.
Pour the batter into the prepared cake pan and spread evenly.
Bake for 40-45 minutes or until a toothpick comes out clean.