Add the blueberries, ground cinnamon (if using), granulated sugar, and water to a saucepan with a lid or a dutch oven.
Bring the mixture to a low boil over medium-high heat, stirring often to dissolve the sugar.
Once the blueberries have released much of their juices, reduce the heat to medium and simmer for 5-10 minutes.
Sift the flour, baking powder, and sugar into a large bowl. Add the cold butter and cut it in with a pastry cutter or two forks until the mixture resembles coarse crumbs (with butter pieces pea-sized).
Add in a small amount of milk and mix to combine. Keep adding a small amount of milk until the mixture comes together into a soft and sticky dumpling dough that can be easily scooped.
Drop the batter on top of the simmering blueberries using a heaping tablespoon, distributing the dough evenly over the surface.
Place the lid on your saucepan (or dutch oven) and allow it to simmer for 15 minutes without lifting the lid.
Once the time is up, lift the lid to check and see that the dough is cooked through.
Serve immediately with a generous dollop of whipped cream or a scoop of vanilla ice cream.