Preheat the oven to 350°F.
Heat a 12-inch cast-iron pan over medium heat. Add the butter to melt. Once melted, add in the peanut butter and remove pan from heat. Using a rubber spatula, add in both sugars and stir until well-combined and a little glossy. Using a whisk, add in the eggs one at a time. Once they’re mixed in, slowly fold in the flour, baking powder, baking soda, and salt with a rubber spatula.