1. Preheat oven to 350°F/180°C and grease a 9-inch round cake pan. For an easy release of the cake later on, line the bottom of the pan with parchment paper, and use a springform pan if you have one.

Make the apple topping:

1. In a small, deep, heavy-bottomed saucepan place butter and sugar. Cook over medium heat until butter is melted, sugar is dissolved, and mixture is bubbling, about 3 minutes. Don’t worry if the mixture Is a bit gritty and not smooth. It would all melt in the oven. Pour into prepared pan and swirl pan until bottom is evenly covered with the caramel.

2. Place the apple slices flat in the pan in a circular pattern, covering most of the caramel layer. Set aside.

Make the cake:

1. In a medium bowl whisk together flour, baking powder, baking soda, and spices. Set aside.

2. In a mixer fitted with the paddle attachment, beat together butter and sugar on medium speed for 3 minutes until fluffy. scrape down the sides and bottom of the bowl as necessary. Add the eggs, one at a time, beating well after each addition. Beat in vanilla extract and orange zest, if using. On low speed, beat in the flour mixture in 3 additions, alternating it with the sour cream in 2 additions, starting and ending with the flour. Do not over mix.
3. Pour batter carefully over apples and smooth the top. Bake for 40-50 minutes, or until a skewer inserted into the center of the cake comes out clean. Allow cake to cool in pan on a wire rack for exactly 15 minutes, then run a knife along the edges of the pan and invert onto a plate. If you used parchment paper, gently remove it. Allow cake to cool completely.
4. Serve slices warm or at room temperature. Leftovers can be kept in the refrigerator for several days.