Preheat your oven to 350°F (175°C). Prepare an 8-inch round cake pan by lining the bottom with parchment paper and spraying the sides with baking spray.

In a small saucepan, dissolve ½ cup of brown sugar in ¼ cup of water over medium heat, stirring until syrupy. Pour this mixture into the prepared pan and arrange sliced Granny Smith apples on top.

In a large bowl, whisk together 1½ cups of all-purpose flour, ½ cup of firmly packed brown sugar, 2 teaspoons of apple pie spice, 1 teaspoon of baking powder, ¾ teaspoon of baking soda, and ½ teaspoon of salt until well combined.

In a separate medium bowl, mix together 2 large eggs, ½ cup of vegetable oil, ¾ cup of sour cream, and ¾ teaspoon of vanilla extract until fully incorporated.

Pour the wet mixture into the dry ingredients and gently stir until just combined. Be careful not to over-mix.

Carefully pour the cake batter over the arranged apple slices in the pan, ensuring even coverage.

Place the pan in the preheated oven and bake for 50-55 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for about 10 minutes, then run a thin knife around the edges before inverting onto a serving plate.