One of the great things about an upside-down cake is the moment when you flip it over and ta-da! there it is–it worked! The fruit is all (or mostly) there and it looks, well, good enough to eat. This cake can be made with all apples, or all plums, as well as with pears, apricots, peaches, nectarines, blueberries, and, of course, the traditional pineapple, though this latter is outside our purview.
6 T butter, in pieces
1 1/2 t lemon zest, divided
1/2 – 3/4 c (light brown) sugar, divided
1 1/2 c quartered prune plums (pitted of course)
1 1/2 c sliced or chopped (peeled) cored apples
3/4 c flour, white or whole grain or a combination
3/4 t baking powder
2 T lemon juice
2 t vanilla extract
1/2 t cinnamon
1. Preheat the oven to 400°F. Put the butter in a 9-inch cast iron skillet or 9-inch cake pan or 8-inch by 8-inch square baking pan and transfer to the oven to melt. This should take 4 minutes or so. Watch that the butter doesn’t burn. Remove from oven and, carefully, spoon off 2 T of the melted butter. Reserve this butter.
2. To the remaining butter in the pan, add 1 t of the lemon zest and1/4 c of the sugar. Stir to blend. Add the fruit, and toss to coat. Spread the fruit evenly over the bottom of the pan.
3. Combine the flour and baking powder in a bowl. Set aside.
4. Beat the eggs with the reserved butter, the remaining 1/2 t of lemon zest, the rest of the sugar, the lemon juice, vanilla extract, and cinnamon, either by hand or with an electric mixer, until light and thick. Add the flour mixture and beat just until incorporated. Spread the batter over the fruit.
5. Bake for about 25 minutes, until golden brown and a toothpick comes out clean. Put on oven mitts. Immediately cover with a serving plate and invert the cake onto the plate. If any pieces of fruit remain in the pan put them back in their rightful place. Serve warm or at room temperature.