Preheat the oven to 350°F (175°C). Generously grease a 9-inch round cake pan.
Pour the melted butter evenly over the bottom of the pan. Sprinkle the brown sugar over the butter.
Arrange the apple slices over the brown sugar, slightly overlapping them. Scatter the cranberries between and around the apple slices.
Sprinkle the cinnamon and nutmeg over the fruit, then drizzle with vanilla extract.
In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt.
In a separate large bowl, beat the softened butter and granulated sugar for about 2 to 3 minutes, until light and creamy.
Add the eggs one at a time, mixing well after each addition. Mix in the vanilla.
Add half of the dry ingredients, followed by the milk and sour cream. Add the remaining dry ingredients and mix just until combined.
Carefully spoon the batter over the apples and cranberries. Spread it evenly without disturbing the fruit underneath.
Bake for 40 to 50 minutes, or until the center is set and a toothpick inserted into the cake comes out clean.
Allow the cake to cool in the pan for 10 to 15 minutes. Run a knife around the edge, place a serving plate over the pan, and carefully flip the cake.
Slowly remove the pan and allow the cake to cool slightly before slicing.