Preheat oven to 350°F.
In a small bowl, combine the flour, baking powder, baking soda and salt; set aside.
In the bowl of a stand mixer, cream the butter and sugar until light and fluffy.
Add the egg, sour cream and vanilla and mix until well-combined.
Add the dry ingredients and mix.
Slowly pour the milk into the batter and mix until just combined.
Pour into the pie plate over the peaches and spread to an even layer.
Bake for 20 minutes, then cover with aluminum foil (you'll want to tent it - don't cover too tightly). If the cake sticks to the foil, don't worry about it....remember, it's an upside down cake! ;)
Bake an additional 20-30 minutes or until a toothpick in the center comes out clean.
Let cool for 5 minutes, then run a butter knife around the edge of the pie pan and flip over onto a serving platter.
Enjoy warm or room temperature with a dollop of whipped cream, if desired!