2 large Granny Smith apples, peeled, cored and sliced
5 Zwieback toasts, (1 1/2 ounces)
3/4 cup all-purpose flour
1/4 teaspoon salt
3 large eggs
2 large large egg whites
1 teaspoon vanilla extract
Preheat oven to 350°F.
Soak cranberries in Calvados (or cider) in a small bowl until softened, about 10 minutes.
Use 2 teaspoons butter to thickly coat bottom and lightly coat sides of a 9-inch square or 7-by-11-inch baking pan. Spread 1/2 cup brown sugar evenly in bottom of pan. Make 3 rows of apple slices over sugar, overlapping them slightly. Sprinkle with reserved cranberries.
Melt remaining 4 teaspoons butter. Set aside.
Finely grind Zwieback in a food processor. Add flour and salt and process until well combined.
Combine eggs, egg whites, vanilla and remaining 1/2 cup brown sugar in a large mixing bowl. Beat with an electric mixer on high speed until thick and pale, about 5 minutes. With mixer at low speed, add reserved Zwieback mixture and melted butter, beating until just combined. (Do not overmix.) Spoon over the fruit in prepared pan.
Bake cake until top is golden and springs back when touched lightly, 30 to 35 minutes. Transfer pan to a wire rack and let cool for 5 minutes. Invert cake onto a wire rack and let cool completely.