Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish or line it with parchment paper for easy removal.
In a medium bowl, combine the chopped rhubarb, 1/2 cup of sugar, cornstarch, vanilla extract, and salt. Mix well and set aside.
In another bowl, whisk together the melted butter, remaining 1/2 cup of sugar, flour, baking powder, eggs, and milk until smooth.
Pour half of the batter into the prepared baking dish and spread it evenly.
Spoon the rhubarb mixture over the batter, then pour the remaining batter on top, spreading it gently.
Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Allow to cool in the pan for about 15 minutes before slicing into bars.
Serve warm or at room temperature. For best results, chill in the refrigerator for a couple of hours before serving.