Making the Nectarine Upside-Down Cake is a straightforward process. Follow these steps for the best results:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan.
2. Prepare the Nectarines: In a skillet, melt 1/2 cup of butter over medium heat. Add the brown sugar and stir until dissolved. Remove from heat and pour the mixture into the prepared cake pan.
3. Arrange Nectarines: Lay the sliced nectarines in a circular pattern over the caramel mixture, ensuring they are evenly distributed.
4. Mix Dry Ingredients: In a bowl, whisk together the flour, baking powder, and salt.
5. Combine Wet Ingredients: In another bowl, beat the granulated sugar and eggs until light and fluffy. Add the milk and vanilla extract, mixing well.
6. Combine Mixtures: Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
7. Pour Batter: Carefully pour the batter over the arranged nectarines in the cake pan.
8. Bake: Place the cake in the preheated oven and bake for 35-40 minutes or until a toothpick inserted in the center comes out clean.
9. Cool: Once baked, remove the cake from the oven and let it cool in the pan for about 10-15 minutes.
10. Invert the Cake: Carefully run a knife around the edges of the cake. Place a serving plate over the cake pan and flip it over to release the cake onto the plate, revealing the nectarines on top.

By following these steps, you will create an incredible Nectarine Upside-Down Cake that is sure to impress!