Rhubarb Dump Cake
Ingredients:
4 cups fresh rhubarb, diced (about 1 to 1¼ pounds)
1½ cups granulated sugar
2 tablespoons cornstarch
1 teaspoon vanilla extract
1 cup water, divided
15.25 ounces yellow cake mix, super moist (such as Betty Crocker)
1 cup unsalted butter, melted