Preheat the oven to 400 degrees F. Add 4 tablespoons butter to a 12-inch oven-safe skillet set over medium heat. Allow the butter to brown until it smells toasted and is a deep golden brown, stir often, about 3-4 minutes. Remove from the heat and add the peaches, 1/2 cup brown sugar, bourbon - if using, and vanilla, tossing to combine. Transfer to the oven and bake 10-15 minutes. Reduce the oven to 375 degrees F. Meanwhile, in a large bowl, whisk together the flour, ground pecans, baking powder, cinnamon, and the remaining 1/2 cup brown sugar. Add 1 stick melted butter and the milk, mix until just combined. Remove the peaches from the oven and dollop the batter over the peaches. Dust with cinnamon sugar. Transfer to the oven and bake for 25-30 minutes, or until golden on top. Let cool 5 minutes. Serve warm or at room temp with ice cream.