Part cake, part pudding, this tender apple and pear upside-down cake is suffused with vanilla bean and the flavors of perfectly ripe apples and pears. Recipe adapted from Cook’s Illustrated.
In a large stainless steel or enamel skillet (not cast iron), gently heat 4 tablespoons butter, until bubbly and fragrant. Remove from heat, stir in vanilla beans and pod, and set aside 10 minutes or more while you prepare the fruit.
Preheat oven to 350 degrees F. Core and cut pears into 1/2-inch thick slices. Core and cut apples into 1/4-inch slices. Remove vanilla pod from pan and gently rewarm butter. Add pears and cook over medium heat 5 or so minutes, until they just start to brown. Add apples, brown sugar, and lemon, and stir gently while sugar dissolves, cooking 1 - 2 minutes. Remove from heat and set aside while you make cake batter.
In a smallish bowl, combine flour, baking powder, and salt. In a medium bowl, fluff room temperature butter with your beater, or a paddle attachment if using a stand mixer. Add sugars and beat until fluffy. Stir in eggs just until combined, then add sour cream and vanilla. Gradually stir in flour.
12.30.09 Update: I made this cake again over the holiday and had remarkably different results. In the first place, I had relatively unripe pears this time and I cooked them for much longer so that they were rather overcooked by the time I added the batter. The end result was a much moister cake, bordering on a pudding. It was still really good, though. If you want a fluffier cake and fruit with bite, use just ripe fruit, and sauté for the short five minutes over medium to medium high heat (just so the butter doesn't smoke). If you want a more dense pudding-like cake, cook the fruit longer until the edges are quite soft. In both cases, remove from oven when a toothpick inserted in the middle comes out clean. It will take longer for the second cake to cook, look for it to set in the middle.