Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan.

In a medium saucepan, melt 1/4 cup of unsalted butter over medium heat. Add 1 cup of brown sugar and stir until the mixture is bubbly and smooth, about 3–5 minutes.

Pour the caramel mixture into the prepared cake pan, and arrange apple slices on top.

In a large mixing bowl, whisk together 1 1/2 cups of all-purpose flour, 2 teaspoons of baking powder, 1/2 teaspoon of baking soda, a pinch of salt, 1 teaspoon of ground cinnamon, and 1/4 teaspoon of ground nutmeg.

In another bowl, cream together 1/2 cup of softened unsalted butter, 1/2 cup of granulated sugar, and 1/4 cup of brown sugar using an electric mixer until light and fluffy.

Add 2 large eggs and 1 teaspoon of vanilla extract to the butter mixture, beating well after each addition.

Mix in 1/2 cup of sour cream and 1/4 cup of milk until combined. Gradually fold in the dry ingredients.

Pour the batter over the arranged apples and bake for 40–45 minutes, or until a toothpick inserted comes out clean.

Let the cake cool in the pan for 10–15 minutes. Carefully run a knife around the edges and flip onto a serving plate.

Serve warm, optionally with vanilla ice cream.