Preheat the oven to 350°F (175°C). Grease a 9-inch round cake pan.

In a saucepan over medium heat, melt the vegan butter and stir in the brown sugar. Cook until the sugar dissolves and forms a smooth caramel.

Pour the caramel into the prepared cake pan and arrange the apple slices over it in a circular pattern.

In a large mixing bowl, combine the flour, baking powder, baking soda, cinnamon, nutmeg, and salt, whisking until evenly blended.

In a separate bowl, combine the plant-based milk and apple cider vinegar. Let it sit for 5 minutes to curdle and form vegan buttermilk.

Stir the granulated sugar, oil, and vanilla extract into the buttermilk mixture.

Slowly incorporate the wet ingredients into the dry mixture, stirring until everything is well blended. Do not overmix.

Pour the batter over the apple slices in the cake pan, spreading it evenly with a spatula.

Bake for 35-40 minutes, or until a toothpick inserted into the center comes clean.

Let the cake cool in the pan for 10 minutes, then carefully run a knife around the edges to release it.

Place a plate over the pan, invert it, and gently lift it to reveal the caramelized apple topping.