Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan with non-stick spray.

In a medium saucepan over medium heat, melt together ½ cup of brown sugar and ¼ cup of unsalted butter, stirring until smooth. Pour the syrup into the prepared cake pan.

Evenly distribute 2 cups of blueberries over the syrup in the pan.

In a large mixing bowl, separate the egg whites from the yolks. Beat the egg whites until stiff peaks form.

Cream ½ cup of softened unsalted butter and 1 cup of granulated sugar until light and fluffy. Add the egg yolks and vanilla extract.

Whisk together 1 ½ cups of flour, 2 teaspoons of baking powder, and ½ teaspoon of salt. Gradually add to the butter mixture alternately with ¾ cup of milk.

Fold beaten egg whites into the batter gently until just combined.

Pour the batter over the blueberries in the cake pan and bake for 55-60 minutes, or until a toothpick comes out clean.

Let cool in the pan for about 15 minutes before inverting onto a serving plate.

Serve warm or at room temperature, optionally with ice cream or whipped cream.