Plum Berry Upside-Down Cake

This recipe is taken from David Lebovitz's book Room for Dessert. Elizabeth made it last summer and we thought it delicious. Lebovitz calls for huckleberries, but we substituted blackberries. Any berries but strawberries may be used.

Plum Berry Upside-Down Cake (1 9"round cake or 1 10"square cake)
Preheat oven to 350 deg. with rack in the middle.
The dessert is best served warm.

Topping:
3 Tbls. butter
3/4 cup light brown sugar
1 cup berries (blueberries can be used, also)
6-8 medium plums, about 1#

Melt butter in a cake pan, directly on the stove top over low heat. Add the brown sugar and stir until sugar is thoroughly moistened. Remove from heat and cool briefly.
Sprinkle half the berries evenly over the moistened brown sugar. Cut the plums in half, remove the pits and cut the plum halves into 1/2" slices. Arrange the slices over the berries in a concentric, overlapping circle. You may also scatter them evenly. Strew the rest of the berries over the plum slices.

Cake:
1 stick butter at room temperature
3/4 cup sugar
1 tsp. vanilla
2 eggs
1 1/2 cup flour
1 1/2 tsp. baking powder
1/4 tsp. salt
1/2 cup milk at room temperature

Beat together the butter and sugar until light and fluffy, 3 to 5 minutes in a stand mixer. Stop the mixer once and scarpe down the sides of the bowl to make sure the butter is completely incorporated. Add the vanilla.
Beat in eggs, one at a time, until completely incorporated.
In a separate bowl, sift together the flour, bkaing powder and salt. Mix half of the flour mixture into the batter. Stir the milk into the batter, add the remaining dry ingredients and stir just until smooth; do not overmix. The batter will be quite thick.
Carefully spead the batter over the fruit in the cake pan, smoothing with a spatula. Bake the upside-cake for about 1 hour, until the top is golden brown and a toothpick inserted in the center comes out clean.
Let the cake stand at least 20 minutes before unmolding. Run a knife around the edge of the cake to loosen it from the pan. Invert a serving plate over the cake and carefully flip over both cake and plate simultaneously. Lift off the cake pan and scrape any fruit that may have stuck to the pan back onto the cake.

Serve with whipped cream or ice cream.