Whisk together the flour, brown sugar, baking powder, cinnamon, and salt in a medium size bowl. Add in the milk and vanilla and mix until fully combined and smooth.
Melt the butter in a 10 inch cast iron skillet over medium heat (this both preheats and greases the pan).
Once melted stir frequently and watch closely for the formation of brown specs on the bottom of the pan.
First the butter will get thick and foamy. Then it will start to bubble rapidly. Don't stop stirring the butter. After a few minutes, the butter will start to turn a golden brown color and you'll start to see those brown specs on the bottom of the pan. It will smell intensely buttery, nutty, and rich.
Once you see the brown specs, remove from the heat and immediately pour into the cake batter (it's okay if a little butter stays behind) and stir to combine.
Pour the batter into the hot greased pan and smooth into an even layer. Then top with the sliced peaches.
Bake the skillet cake at 350°F for 30-40 minutes or until the batter is fully set and the top is browned.
Dust the peach skillet cake with powdered sugar and serve warm with whipped cream or vanilla ice cream!
Store covered for up to 2 days. Substitute any stone fruit or berries!