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. Toss to coat the rhubarb evenly.

Pour half of the cake batter into the prepared pan. Spread the rhubarb mixture over the batter, then top with the remaining batter.

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

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