Preheat an oven to 350 F.
Mix the crisp topping. In a mixing bowl, combine the dry ingredients and the pecans and use a fork to mix it together. Pour in the melted butter and use a fork to mix until clumps form. Transfer to the fridge while you make the filling.
Mix the filling. In a large mixing bowl, gently toss the peaches with the sugar, cornstarch, bourbon, maple syrup, vanilla, and salt until well mixed.
Assemble. Set the cast iron skillet on a burner over medium heat. Melt the remaining butter and swirl it around so it coats the pan. When it's hot and foamy, pour in the peaches and any juices and spread them in an even layer. Remove the crisp topping from the fridge and sprinkle on top.
Bake. Bake for 40-45 minutes, or until the top is golden brown and the peach filling is bubbly. Remove the peach crisp from the oven and rest for 30 minutes before serving warm with scoops of vanilla ice cream or whipped cream.