Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Prepare the filling: In a large bowl, combine the chopped rhubarb, 1 cup of sugar, 1/4 cup of flour, vanilla extract, and cinnamon. Toss gently until the rhubarb is evenly coated. Pour the mixture into the prepared baking dish and spread into an even layer.

Prepare the topping: In a separate medium bowl, whisk together the 1 1/2 cups flour, 1/3 cup sugar, baking powder, and salt.

Cut in the butter: Using a pastry blender or your fingertips, cut the cold, cubed butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.

Add liquid: Pour in the milk (or buttermilk) and stir just until the dough comes together. Do not overmix. Drop spoonfuls of the biscuit dough evenly over the top of the rhubarb filling, leaving small gaps between dollops for steam to escape.

Bake: Brush the tops of the biscuit dough lightly with the beaten egg, if desired, for extra golden color. Bake for 45 to 50 minutes, or until the topping is golden brown and the rhubarb juices are bubbly and thickened.

Cool and serve: Let the cobbler cool for at least 15 minutes before serving warm, preferably topped with a scoop of vanilla bean ice cream or fresh whipped cream.