Preheat the oven to 350°F (175°C).
Prepare the rhubarb: Toss the chopped rhubarb with 1/4 cup of sugar and let it sit for 15 minutes to release excess moisture. Drain the rhubarb.
Mix wet ingredients: In a bowl, whisk together eggs, melted butter, milk, vanilla extract, and the remaining sugar.
Add dry ingredients: Fold in the flour, cornstarch (if using), salt, and cinnamon until just combined.
Combine: Gently fold the rhubarb into the batter.
Pour the mixture into a greased 9-inch pie dish or baking dish.
Bake for 40-45 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Cool for 15-20 minutes before slicing. Optionally, serve with whipped cream or vanilla ice cream.