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

Melt together brown sugar and butter in a medium saucepan over medium heat until syrupy, about 3-4 minutes.

Pour the syrup evenly into the bottom of the prepared pan, and arrange blueberries in a single layer over the syrup.

Whip egg whites until stiff peaks form in a large mixing bowl, about 5-7 minutes, and set aside.

Cream softened butter and granulated sugar until light and fluffy, then mix in egg yolks and vanilla extract.

Whisk together flour, baking powder, and salt in a separate bowl, then gradually add to the butter-sugar mixture with milk, alternating dry and wet ingredients.

Fold the whipped egg whites into the batter gently to maintain airiness.

Pour the batter over the blueberries without disturbing the fruit layer.

Bake the cake for 55-60 minutes, or until a toothpick comes out clean, with the top golden brown.

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