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

Prepare the topping: Pour the 1/2 cup melted butter into the prepared cake pan. Sprinkle the brown sugar evenly over the butter. Arrange the sliced nectarines decoratively over the sugar and butter mixture, overlapping slightly.

Prepare the batter: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a separate large bowl, cream together the softened butter and granulated sugar until light and fluffy (about 3-4 minutes). Beat in the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the flour mixture. Mix until just combined; do not overmix.

Carefully spoon the cake batter evenly over the nectarines in the pan. Bake for 45 to 50 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cake cool in the pan on a wire rack for exactly 10 minutes. Place a large serving plate upside down over the cake pan. Holding both firmly, carefully flip the cake pan and plate over. Let the pan rest for a minute, then gently lift the pan, allowing the caramelized nectarines to release onto the plate.