1In a large mixing bowl cream together the 1 cup of butter and sugar with an electric mixer or stand mixer until light and fluffy
2Beat in the vanilla, then add the eggs one at a time.
3Turn the mixer to low speed and mix in the buttermilk until just combined.
4The mixture will look curdled. Using a sifter or fine-mesh strainer, combine the flour, baking powder, baking soda and salt in a separate medium bowl. Stir in 3 stages, inot the wet mixture until combined. Set aside.
5Place the 1/2 cup butter into a 9 X 13 inch baking pan and melt in the oven. Remove pan from the oven and stir in the brown sugar and oats.
6Spread evenly throughout the bottom of the pan.
7Layer the peach slices evenly across the pan on top of the sugar/oat glaze.
8Pour the cake mixture over top of the layered peaches. Spread evenly with a spatula.
9Tap the pan several times on the counter top to rid any air bubbles. Bake in the preheated oven for 45 to 55 minutes or until a toothpick inserted into the center comes out clean.
10Remove from oven, and immediately invert onto a large serving plate. Allow to cool, then serve.