Step 1: Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan thoroughly.
Step 2: Melt 4 tablespoons of butter and pour it into the bottom of the cake pan. Sprinkle the brown sugar evenly over the butter.
Step 3: Arrange the nectarine slices in a decorative circular pattern over the brown sugar layer, overlapping them slightly if necessary.
Step 4: In a large bowl, cream together the remaining butter and granulated sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract.
Step 5: In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the butter mixture, alternating with the milk, and mix until just combined.
Step 6: Carefully spread the batter over the nectarines in the pan, smoothing the top with a spatula.
Step 7: Bake for 40-45 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Step 8: Remove from the oven and let cool in the pan for 10 minutes. Run a knife around the edge of the cake, place a serving plate on top, and carefully invert the cake.
Step 9: Serve warm or at room temperature.