Over low heat, melt the butter in the bottom of a 9-inch round cake pan, stirring in the brown sugar until it dissolves. Swirl the pan to coat the bottom, then remove from the heat and cool. Cut the plums into ¼-inch-thick wedges and arrange them neatly over the brown sugar mixture, covering the bottom completely. Set the pan aside and prepare the cake batter.

Preheat the oven to 350°F.

With an electric mixer, cream the butter with the sugar until pale, light, and fluffy. Add the vanilla extract and beat in the egg yolks one at a time, scraping the bowl to make sure all the butter is incorporated. Sift together the flour, baking powder, and salt, and add them to the batter alternately with the milk, beginning and ending with the dry ingredients. Whisk the egg whites with the cream of tartar until they are stiff enough to hold a slight shape. Fold the whites into the batter a third at a time. This helps to lighten the batter before all the egg whites are incorporated. Pour the batter into the prepared pan and spread it evenly over the plums. Bake for 25 to 35 minutes, until the top is browned and the cake pulls away slightly from the edges of the pan. Let the cake cool for 15 minutes before turning it out onto a cake plate. Serve with slightly sweetened whipped cream, flavored, if you wish, with a plum eau-de-vie.

Variations: This cake is very versatile and can be made with pears and peaches, but it is better made with a slightly acidic fruit like cranberries. Cranberry upside-down cake is not only beautiful in appearance but the berries’ tartness serves as a counterbalance to the sweet topping.