Melt 4 tablespoons unsalted butter in a small pan. Add brown sugar and stir until it dissolves. If mixture if having trouble dissolving, try adding a teaspoon or two of water to help it along. Pour brown sugar mixture into your baking dish so it covers the bottom completely. Arrange plum wedges over the brown mixture sot hat the bottom of the pan is completely covered. Set aside.

Preheat oven to 350F. In an electric mixer, combine butter and sugar until light and fluffy. Add vanilla, and egg yolks one at a time.

Sift together dry ingredients which include flour, baking powder, and salt, in a separate bowl. Beat in half of the flour mixture to mixing bowl, then the milk, and finally the other half of the flour mixture.

In a separate bowl, whisk the eggs whites and cream of tartar until slightly stiff. Gently fold into the cake batter one third at a time.

Pour the batter onto the prepared pan and spread evenly over the plum topping. Bake for 30-35 minutes, or until the top of the cake is browning, the middle is cooked through, and the cake is pulling way slightly from the pan edges. Let the cake cool in the pan for 15 minutes before turning out onto serving platter or cake plate. Add a dollop of whipped cream to each slice of cake (optional).

Whisk on high speed whipping cream, powdered sugar and a pinch of salt until medium to stiff peaks form, about 1 minute.