Heat oven to 350°. In a small saucepan, melt 4 tablespoons of the butter over medium heat. Stir in the brown sugar and cook until molten and partially dissolved. Pour the sugar into a 9-inch round cake pan, and then add the blueberries, spreading them evenly across the pan.
In a medium bowl, whisk the flour with the baking powder and salt. In a large bowl, beat the remaining 8 tablespoons butters and the granulated sugar with a handheld mixer on medium-high speed until pale and fluffy, about 3 minutes. Add the vanilla and egg yolks, and beat until evenly combined. Add the dry ingredients followed by the milk and beat on low speed until a smooth batter forms.
In a clean bowl, beat the egg whites and cream of tartar with a handheld mixer on medium-high speed until they form stiff peaks. Using a large rubber spatula, fold the egg whites into the cake batter until just combined, and then spread the batter over the blueberries in the pan, smoothing the top.
Bake until a toothpick inserted into center of cake comes out clean, about 1 hour. Transfer the cake to a rack and let cool for 5 minutes. Place a serving platter over the pan and invert the two together. Let the pan sit upside down on the platter for 5 minutes, and then remove the pan. Let the cake cool completely on the platter, and then spoon whipped cream over the cake and dust with grated nutmeg before serving.