Peach Upside Down Cake
1/2 cup sliced almonds, toasted
2 tablespoons unsalted butter, plus 5 tablespoons melted and cooled
1 cup sugar
1 lb peaches, halved, pitted, and cut into 3/4-inch wedges, or 12 oz frozen sliced peaches, thawed and drained
1 cup all-purpose flour
1 teaspoon baking powder
1/8 teaspoon baking soda
1/8 teaspoon ground nutmeg
1/2 cup whole milk
1 large egg plus 1 large yolk
1 teaspoon grated lemon zest
1 teaspoon vanilla extract
Set oven rack to middle position and heat oven to 350 degrees. Pulse almonds in the food processor until finely ground.
Melt 2 tablespoons butter in 10-inch cast-iron skillet over medium heat. Add 1/4 cup sugar and pinch of salt. Cook, whisking constantly, until sugar is melted, smooth, and deep golden brown, about 4 minutes. Off heat, carefully arrange peaches cut side down in tight circle around edge of skillet, overlapping as needed. Arrange remaining peaches in center of skillet.
Whisk flour, baking powder, baking soda, nutmeg, 1/4 teaspoon salt, and ground almonds together in large bowl. In separate bowl, whisk milk, egg and yolk, lemon zest, vanilla, remaining 6 tablespoons melted butter, and remaining 3/4 cup sugar together until smooth. Stir milk mixture into flour mixture until just combined.
Pour batter over peaches and spread into even layer. Transfer skillet to oven and bake until cake is golden brown and toothpick inserted into center comes out clean, 25 to 30 minutes, rotating halfway through baking.
Using potholders, transfer skillet to wire rack and let cake cool for 10 minutes. Being careful of hot skillet handle, run paring knife around edge of cake to loosen. Place wire rack over skillet, and holding each tightly, invert cake onto rack and let sit until cake releases itself from skillet, about 1 minute. Place rack over baking sheet to catch drips Remove skillet; gently scrape off any peaches stuck in skillet and arrange on top of cake. Let cake cool completely, about 1 hour. Serve.