Step 1

Make the topping: Melt butter in the bottom of a 10 1/2-inch cast-iron skillet over low heat. Add brown sugar, stirring until dissolved. Swirl to coat the bottom; remove from heat, and cool. Cut pears into 1/4-inch-thick wedges, and arrange them in a circular pattern over the brown-sugar mixture to cover completely; set aside.

Step 2

Make the batter: Preheat the oven to 350 degrees. In a medium bowl, whisk together flour, baking powder, and salt; set aside.

Step 3

In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar on medium speed until light and fluffy, about 2 minutes. Add vanilla extract; beat to combine. Add egg yolks, one at a time, beating to combine.

Step 4

Alternating with the milk, gradually add the flour mixture to the butter mixture, and mix, on low speed, just until the flour is incorporated.

Step 5

In a large bowl, beat reserved egg whites and the cream of tartar with a hand mixer until stiff but not dry. Using a rubber spatula, fold egg whites into the batter. Transfer to skillet. Using an offset spatula, spread the batter evenly, being careful not to disturb the pears.

Step 6

Bake until well browned on top and a cake tester inserted into the center comes out clean, about 45 minutes. Run a knife around inside of pan, and immediately invert the cake onto a serving dish. Serve warm or at room temperature.