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

In a mixing bowl, cream together the softened butter and sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add to the butter mixture, mixing until just combined.

In another bowl, combine the chopped rhubarb, sugar, and cornstarch. Mix well to coat the rhubarb.

Pour half of the cake batter into the prepared pan. Spread the rhubarb mixture evenly over the batter.

Top with the remaining cake batter, spreading it evenly over the rhubarb.

Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean.

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