Heat oven to 400°F. Rub the bottom and sides of a 9- to 10-inch baking pan or oven-safe dish (such as a cast-iron skillet) with butter.

First, make the rhubarb mixture: In a medium saucepan over medium-high heat, stir together chopped rhubarb, 3/4 cup sugar, cornstarch and water. (If using frozen rhubarb, start cooking on low heat to gently defrost before increasing to medium-high.) Bring mixture to a boil. Cook and stir for 1 minute, then remove from heat; transfer to the prepared baking pan. Sprinkle evenly with diced butter and cinnamon.

Next, make the biscuit topping: In a medium bowl, whisk together flour, 1 tablespoon sugar, baking powder and salt. Use a pastry blender, two knives or your fingers to cut in cold butter until the mixture resembles coarse crumbs.

Stir in milk and egg just until a shaggy dough forms. Spoon 1-tablespoon-size dollops of biscuit topping evenly over rhubarb mixture. Sprinkle the remaining 2 tablespoons of sugar evenly over the biscuits and rhubarb.

Bake 20 to 25 minutes or until biscuits are golden-brown and rhubarb mixture is bubbly. Cool at least 10 minutes before serving. Serve warm or at room temperature.