Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

Add the softened butter, eggs, vanilla extract, and buttermilk. Mix until well combined and smooth.

In a separate bowl, combine the chopped rhubarb, sugar, and cornstarch. Stir until rhubarb is coated.

Fold the rhubarb mixture into the cake batter gently.

Pour the batter into the prepared cake pan and smooth the top.

Bake for 35-40 minutes or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 10 minutes before transferring to a wire rack to cool completely.