Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a medium bowl, combine chopped rhubarb, 1/2 cup granulated sugar, cornstarch, and 1 tsp vanilla extract. Toss gently to coat. Set aside.

In a large bowl, cream together the softened butter and 1 1/2 cups granulated sugar until light and fluffy.

Beat in the eggs one at a time, then stir in 1 tsp vanilla extract.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined. Do not overmix.

Spread half of the cake batter evenly into the prepared baking pan. Spoon the rhubarb mixture over the batter, then dollop the remaining cake batter over the rhubarb. Use a knife or spatula to gently swirl the batter, ensuring most of the rhubarb is covered.

Bake for 45-50 minutes, or until a wooden skewer inserted into the center comes out clean. The topping should be golden brown.

Let the cake cool in the pan on a wire rack for at least 20 minutes before slicing and serving. This cake is delicious served warm or at room temperature.