In a medium saucepan, combine the chopped rhubarb, 3/4 cup of granulated sugar, cornstarch, salt, and water. Cook over medium heat, stirring occasionally, until the rhubarb is soft and the mixture thickens, about 10 minutes. Remove from heat and stir in the vanilla extract. Set aside to cool slightly.

Preheat the oven to 350°F. Grease an 8x8-inch baking dish.

In a mixing bowl, cream together the softened butter and remaining 1/4 cup of granulated sugar until light and fluffy. Beat in the egg.

In another bowl, whisk together the flour and baking powder. Gradually add the dry ingredients to the butter mixture, alternating with the milk, until just combined.

Spread half of the batter into the prepared baking dish. Pour the rhubarb mixture over the batter, then dollop the remaining batter on top. Use a knife to swirl the batter slightly into the rhubarb.

Bake for 35-40 minutes, or until the top is golden and a toothpick inserted in the center comes out clean. Allow to cool in the pan before cutting into squares.