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1
Preheat the oven to 350F (175C).
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2
To make the topping, put the 3 tablespoons (1 1/2 ounces/45 g) butter in a 9-inch (23-cm) round cake pan or cast iron skillet.
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3
Set the pan directly on the stovetop over low heat until the butter melts.
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4
Add the brown sugar and stir until the sugar is thoroughly moistened.
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5
Remove from the heat and let cool briefly.
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6
Distribute half of the blueberries evenly over the brown sugar mixture in the pan.
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7
Arrange the plum slices over the blueberries in overlapping concentric circles, or just scatter them in an even layer.
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8
Strew the remaining blueberries on top of the plums.
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9
Set aside.
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10
To make the cake, in a small bowl, whisk together the flour, baking powder, and salt.
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11
In a stand mixer fitted with the paddle attachment (or in a bowl by hand), beat together the 1/2 cup (4 ounces/115 g) butter and the sugar on medium speed until the mixture is light and fluffy, 3 to 5 minutes.
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12
Add the vanilla and eggs, one at a time, beating until completely incorporated.
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13
Gradually mix in half of the flour mixture.
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14
Stir in the milk followed by the rest of the flour mixture and mix just until combined.
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15
Scrape the batter on top of the fruit in the pan and smooth it into an even layer.
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16
Bake until golden brown and a toothpick inserted in the center of the cake comes out clean, about 1 hour.
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17
Let cool for about 15 minutes.
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18
Run a knife around the sides of the cake to help loosen it from the pan.
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19
Invert a serving plate over the pan.
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20
Wearing oven mitts, grasp both the pan and the plate and turn them over together.
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21
Carefully lift off the pan.
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22
Serve the cake warm with whipped cream (page 239) or your favorite flavor of ice cream.
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23
The cake will keep for 2 days at room temperature, but since its best served warm, you can rewarm the cake, wrapped in aluminum foil, in a low oven, or reheat slices in a microwave oven.
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24
You can use fresh apricots instead of plums, and substitute any kind of berries (except strawberries) for the blueberries.
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25
In the winter, replace the stone fruit and berries with 2 cups (8 ounces/225 g) fresh or frozen unthawed cranberries.
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26
A great do-ahead trick for just about any type of upside-down cake is to make the cake, invert it onto a serving platter, and leave the inverted pan in place.
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27
The cake will stay warm for about an hour this way.
