Preheat the oven to 375 degrees Fand coat a 9-inch-square cake pan with cooking spray. Combine 3/4 cup sugar, 1/2 teaspoon fine salt and 1 1/2 tablespoons water in a medium saucepan and stir until the mixture looks like wet sand. Cook over medium-high heat, gently swirling the pan occasionally but not stirring, until light amber, about 5 minutes. Remove from the heat and immediately pour into the prepared cake pan (the caramel hardens quickly). Arrange the nectarines over the caramel in 4 rows; set the pan aside.
Whisk the flour, baking powder, baking soda and the remaining 1/2 teaspoon fine salt in a bowl. Beat the butter and the remaining 1 cup sugar in a large bowl with a mixer on medium-high speed until light and fluffy, about 3 minutes. Add the eggs one at a time, beating after each addition, then beat in the vanilla. Reduce the mixer speed to low and add the flour mixture in 3 batches, alternating with the buttermilk, beginning and ending with the flour, and mix until just incorporated. Pour the batter over the nectarines and spread evenly.
Transfer to the oven and bake until the cake is golden and a toothpick inserted into the center comes out clean, about 50 minutes. (Cover loosely with foil if the cake is browning too quickly.) Let sit 15 minutes, then invert onto a platter and let cool completely. Sprinkle with sea salt.
Photograph by Con Poulos
Courtesy of Food Network Magazine