Here's the recipe with his tweaks in bold:
Apple Upside-Down Cake from Cook's Illustrated slightly tweaked by Andrew
Butter the bottom and sides of a 9-in round, 2-in deep cake pan. Adjust the oven rack to the lowest position and heat to 350 degrees.
4 Tbsp unsalted butter, cut into 4 pieces, plus extra for the pan
4 Granny Smith apples (about 2 pounds), peeled and cored
2/3 cup packed light brown sugar
2 tsp fresh lemon juice
1 cup all-purpose flour
1/4 cup finely ground almonds
1 tsp baking powder
1/2 tsp salt
3/4 cup sugar
1/4 cup packed light brown sugar
2 large eggs
6 Tbsp unsalted butter, melted and slightly cooled
1/2 cup sour cream
1 tsp almond extract
2 tsp grated orange zest
1. For the topping: Halve apples from end to end. Cut 2 apples into 1/4 inch-thick slices, set aside. Cut the remaining 2 apples into 1/2 inch-thick slices. Heat butter in a heavy skillet over medium heat. Add the 1/2 inch-thick apple slices and cook, stirring frequently until they begin to caramelize (5-6 min.). Do not cook them until done! Add the 1/4 inch-thick apples, brown sugar, and lemon juice and continue cooking, stirring constantly, until sugar dissolves and apples are coated--about 1 minute longer. Transfer the mixture to the prepared pan and lightly press into an even layer.
2. For the cake: Whisk flour, ground almonds, baking powder and salt together and set aside. Whisk granulated sugar, brown sugar, and eggs together in a large bowl until thick and homogeneous, about 1 minute. Slowly whisk in melted and slightly cooled butter until combined. Add sour cream, almond extract and orange zest until just combined. Pour batter into pan and spread evenly over the apples. Bake until the cake is golden brown and a toothpick inserted in the center comes out clean, 35 to 40 minutes.
3. Cool pan on wire rack 20 minutes. Run a knife around sides of cake to loosen. Place the wire rack over the pan and turn over. Lift off the cake pan. Cool completely then transfer to a serving platter.