Preheat your oven to 350°F (175°C). Grease and flour a 9-inch springform pan or standard round cake pan.

In a 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, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.

In a separate medium bowl, whisk together the flour, baking powder, and salt. Alternately add the dry ingredients and the sour cream to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.

Pour the batter into the prepared pan and smooth the top. Arrange the thinly sliced nectarines decoratively over the batter. Sprinkle evenly with coarse sugar, if using.

Bake for 45 to 55 minutes, or until a wooden skewer inserted into the center comes out clean. If the top begins to brown too quickly, loosely tent with foil.

Let the cake cool in the pan on a wire rack for 15 minutes before attempting to remove the sides of the springform pan or inverting onto the rack to cool completely.