In a medium bowl, whisk together the flour, baking powder, and salt.
In the bowl of a stand mixer, or using a large bowl and hand mixer, beat together the butter and sugar on medium speed until light and fluffy, about 3-5 minutes.
Add the eggs one at a time, stopping to scrape the bowl down in between additions. Beat in the vanilla.
Add the dry mixture and buttermilk in three increments, mixing on low speed until fully combined.
Gently dollop and spread the batter over the peaches. Be careful, the more you move things around the more you will change the arrangement of your peaches. Place the cast iron pan on a sheet pan and place on the middle rack of the oven.
Bake for 45-55 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs on it.
Remove from the oven and let cool in the pan for 30 minutes.
Run a butter knife around the edge of the cake in the pan. Unmold the cake by inverting the cast iron pan over a serving plate, with a sheet pan underneath and gently lifting the pan away from the cake. A couple of peaches may slip off, but just put them back in place. Serve with ice cream or whipped cream!