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1
Over low heat, melt the butter in the bottom of a 9-inch round cake pan, stirring in the brown sugar until it dissolves.
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2
Swirl the pan to coat the bottom, then remove from the heat and cool.
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3
Cut the plums into 1/4-inch-thick wedges and arrange them neatly over the brown sugar mixture, covering the bottom completely.
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4
Set the pan aside and prepare the cake batter.
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5
Preheat the oven to 350F.
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6
With an electric mixer, cream the butter with the sugar until pale, light, and fluffy.
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7
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.
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8
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.
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9
Whisk the egg whites with the cream of tartar until they are stiff enough to hold a slight shape.
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10
Fold the whites into the batter a third at a time.
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11
This helps to lighten the batter before all the egg whites are incorporated.
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12
Pour the batter into the prepared pan and spread it evenly over the plums.
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13
Bake for 25 to 35 minutes, until the top is browned and the cake pulls away slightly from the edges of the pan.
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14
Let the cake cool for 15 minutes before turning it out onto a cake plate.
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15
Serve with slightly sweetened whipped cream, flavored, if you wish, with a plum eau-de-vie.
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16
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.
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17
Cranberry upside-down cake is not only beautiful in appearance but the berries tartness serves as a counterbalance to the sweet topping.
