Spread rhubarb into the bottom of a 9x13-inch baking dish.
Beat 3/4 cup white sugar and butter together in a bowl using an electric mixer until creamy and smooth; stir in 1 cup flour, milk, baking powder, vanilla extract, and 1/2 teaspoon salt.
Pour batter over rhubarb.
Mix 1 cup sugar, cornstarch, and 1/4 teaspoon salt in a bowl; sprinkle over batter.
Pour boiling water over sugar mixture layer.
Bake in the preheated oven until cobbler is lightly browned, about 1 hour.