Cal Peternell likes teaching recipes that are extremely versatile, like this one. The cake is wonderful when it's made with almost any type of fruit, from figs and blood oranges to pineapple. Peternell usually uses sweet Meyer lemons from his neighbor's tree. Regular lemons are tasty too and add a bitter note that's a lovely contrast to the gooey brown-sugar topping.
Preheat the oven to 350°. Set a 9-inch nonstick cake pan over moderate heat. Add 4 tablespoons of the butter and when it is melted, stir in the brown sugar until dissolved, about 1 minute. Remove from the heat. Arrange the lemon slices in the melted brown sugar.
In a medium bowl, whisk the flour with the baking powder and salt. In a large bowl, with an electric mixer, beat the remaining 8 tablespoons of butter with the granulated sugar until light and fluffy. Beat in the vanilla and the egg yolks, one at a time. At low speed, beat in the dry ingredients in 3 batches, alternating with the milk.
In a stainless steel bowl, beat the egg whites with the cream of tartar at high speed until firm peaks form. Fold one-third of the beaten whites into the batter, then fold in the rest. Scrape the batter into the prepared pan and bake for about 30 minutes, or until a toothpick inserted in the center of the cake comes out clean. Let cool in the pan for 15 minutes, then invert it onto a plate. Serve warm or at room temperature with whipped cream.