Preheat the oven to 350 degrees F. Grease a 9-inch-square baking pan. I placed a sheet of parchment paper and sprayed with with Pam with Flour.
In a bowl, sift together the flour, baking soda, baking powder and salt. Set aside.
In a small bowl, combine the 1/2 cup sugar and the cinnamon. This is the topping. Set aside.
In the bowl of the KitchenAid with the paddle attachment, cream the butter and sugar together until light and fluffy on medium speed (about 4 minutes). With the mixer on low, add the eggs, one at a time, then the sour cream and vanilla, and mix until the batter is smooth.
With the mixer on low, slowly add the dry ingredients to the batter and mix just until combined.
Spread half of the batter evenly in the pan. Top with half of the nectarines, then sprinkle with two-thirds of the sugar mixture. Spread the remaining batter on top, arrange the remaining nectarines on top and sprinkle with the remaining sugar mixture and the pecans.
Bake the cake for 45 to 55 minutes, until a toothpick inserted in the center comes out clean. Serve warm or at room temperature. My cake took 60 minutes to cook.
Adapted from the Barefoot Contessa's Fresh Peach Cake