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

In a medium saucepan over low heat, melt together ½ cup of brown sugar and ¼ cup of butter until syrupy. Pour into the cake pan and arrange 2 cups of blueberries.

In a separate bowl, beat 3 egg whites until soft peaks form and set aside.

Cream ½ cup of unsalted butter and 1 cup of granulated sugar until light and fluffy. Add 3 egg yolks and 1 teaspoon of vanilla extract.

Mix 1 ½ cups of flour, 1 ½ teaspoons of baking powder, and a pinch of salt, then gradually add to the butter mixture alternating with ½ cup of milk. Stir until just combined.

Fold in the whipped egg whites gently to maintain airiness.

Spoon the batter over the blueberries and bake for 55-60 minutes until golden brown.

Let the cake cool for 15 minutes in the pan, then invert onto a serving platter.