Preheat the oven to 350°F.
Peel, pit, and slice peaches into ½ - 1 inch thick slices.
Mix in a bowl with the remaining filling ingredients. Let sit while you make the snickerdoodle.
Make the cookie dough: Cream butter and sugars until fluffy. Add eggs + vanilla. Stir in dry ingredients + crushed graham crackers until a soft dough forms.
Assemble in skillet: Pour peaches and their juices into a buttered 10–12 inch skillet (or 9x13 baking dish). Taste the peach mixture before baking and adjust sweetness if necessary — fruit ripeness can be fickle.
Dot or gently press cookie dough across the top; some gaps are good. I like to do 1-2 inches apart. Sprinkle cinnamon sugar all over the top.
Bake 35–45 minutes, until the top is golden and the edges are bubbly. Let cool for at least 10 minutes.