Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan, or line the bottom with parchment paper.

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

In a large bowl, cream together the softened butter and granulated sugar using an electric mixer until light and fluffy, about 3-4 minutes.

Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Mix 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 only until just combined; do not overmix.

Pour the batter evenly into the prepared cake pan. Artistically arrange the thin nectarine slices over the top of the batter in concentric circles or a pattern of your choice.

Sprinkle the turbinado sugar evenly over the nectarines (if using). Bake for 40 to 45 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cake cool in the pan for 15 minutes before carefully inverting it onto a wire rack to cool completely.