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

In a medium saucepan, combine 1/2 cup of brown sugar and 1/4 cup of unsalted butter over low heat until syrupy.

Pour this mixture into the bottom of your prepared cake pan and scatter 2 cups of blueberries over the syrup.

Beat the egg whites in a clean bowl until stiff peaks form.

Cream together 1/2 cup of softened butter and 1 cup of brown sugar until light and fluffy; add egg yolks and 1 teaspoon of vanilla.

Whisk together 1 1/2 cups of all-purpose flour, 1 1/2 teaspoons of baking powder, and a pinch of salt.

Gradually add the dry mixture to the butter mixture, alternating with 1/2 cup of whole milk.

Gently fold in the beaten egg whites until no streaks remain.

Pour the batter over the blueberry layer and spread evenly. Bake for 55-60 minutes.

Cool in the pan for 15 minutes before inverting onto a serving platter.