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

In a medium saucepan, combine 1 cup of brown sugar and 1/2 cup of butter over medium heat until bubbling, then pour into the prepared pan and add blueberries.

In a large bowl, beat the egg whites until stiff peaks form. In another bowl, cream together 1/2 cup of butter and 1 cup of sugar until fluffy. Add egg yolks and vanilla extract.

Gradually add flour, baking powder, and salt to the butter mixture, alternating with milk until just combined.

Gently fold the beaten egg whites into the batter until just incorporated.

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

Allow the cake to cool for about 10 minutes before inverting onto a serving platter.

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