Preheat oven to 350 degrees F.

In a large bowl, combine flour, sugar, baking soda and salt.

In a large glass measuring cup or another bowl, whisk together buttermilk, 4 tablespoons butter and eggs. Pour mixture over dry ingredients and stir using a rubber spatula just until moist.

Add remaining 4 tablespoons butter to the bottom of a cast iron skillet or 9-inch cake pan, swirling the pan until the bottom and sides are well coated. Sprinkle brown sugar evenly over the melted butter. Arrange nectarine slices on top in a single layer. Scoop the batter evenly over the nectarines, smoothing out the top.

Place into oven and bake for 45 minutes, or until a tester inserted in the center comes out clean.

Let cool for 5-10 minutes. Run a knife around the edge of the cake and turn upside down onto a serving plate.

Serve immediately.