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

In a medium saucepan, melt together 1 cup of brown sugar and 1/4 cup of butter until syrupy. Remove from heat and pour into the cake pan, then add 2 cups of blueberries on top.

In a mixing bowl, beat the egg whites of 3 eggs until stiff peaks form. Set aside.

Cream together 1/2 cup of softened unsalted butter and 1 cup of granulated sugar. Beat in the egg yolks one at a time, then add 1 teaspoon of vanilla extract.

In another bowl, whisk together 1 1/2 cups of flour, 2 teaspoons of baking powder, and a pinch of salt. Gradually add to the creamed mixture alternating with 1/2 cup of milk.

Gently fold the beaten egg whites into the batter until no streaks remain.

Spoon batter over the blueberry mixture in the cake pan, spreading gently without disturbing the blueberries.

Bake in the preheated oven for 55-60 minutes. Let cool in the pan for about 15 minutes before inverting onto a serving plate.

Allow the cake to settle for 5-10 minutes before serving. Best enjoyed warm or at room temperature.