Heat oven to 425°F and butter pan. Combine blueberries, 1/3 cup sugar and 1/3 cup water in a saucepan and place over high heat. Bring to a boil, then reduce heat to a simmer and cook for 5 minutes. Drain blueberries well; reserve blueberries and return cooking liquid to the pot off the heat.
Sift flour and salt into a mixing bowl. Add butter and blend with fingertips, two knives or a pastry blender until the mixture becomes grainy, a bit coarser than cornmeal. In a small bowl, beat the egg and milk, then add to the dough, stirring all the while. Add vanilla and continue stirring. Lastly, add the baking powder. Mix well and spoon into the cake pan. Scatter the reserved blueberries over the top. Mix remaining 1/3 cup sugar with cinnamon and sprinkle evenly over the top.
Place in oven and immediately reduce heat to 375°F. Cook for 25 to 28 minutes, or until a toothpick in the center comes out with no raw cake dough on it. Let cake cool for at least 15 minutes before serving.
Make the blueberry sauce as the cake rests. Place blueberry cooking liquid over medium-low heat and bring to a simmer. Dissolve cornstarch in ¼ cup of water and drizzle into the simmering juice. Cook until it’s sauce consistency and pass separately with the cake for drizzling. Serves 6 to 8.
