Preheat oven to 350° F. and spray a 9" round cake pan with cooking spray.
In a large bowl whisk together the flours, cinnamon, baking powder, baking soda, and salt. In a separate bowl whisk together the Greek yogurt, sugar, eggs, milk, oil, and vanilla. Set aside.
In a small saucepan melt the butter, brown sugar, and cinnamon together over medium low heat. Stir until the sauce is smooth and the butter and sugar are incorporated, about 2 minutes. Remove the saucepan from the heat and pour into the prepared cake pan. Spread it into and even layer so that it coats the bottom of the pan. Arrange the sliced peaches on top of the sauce.
Finish making the cake batter by stirring the wet ingredients into the dry. Once combined stir in the diced peach. Pour the cake batter on top of the peach slices and sauce.
Bake on the middle rack of the oven for 35-40 minutes or until a toothpick inserted in the center comes out clean.
Remove the cake from the oven and run a knife along the edges. Let the cake cool for 10 minutes in the pan before inverting it onto a serving platter. Once inverted, leave the pan on top of the cake for another minute and then remove it. Use a rubber spatula to scrape out any of the leftover sauce from the pan and spread it on top of the cake. Cool for another 5-10 minutes and then serve.