Preheat oven to 350 ℉.
Spray a 9 x 13 inch baking pan with non-stick cooking spray.
Add 2 cans of peaches and their juice into a large bowl. For the third can of peaches, drain the juice and only add the peaches to the bowl.
Mix in cornstarch, ¼ cup of the brown sugar, cinnamon, pinch of salt, and vanilla extract.
Add the mixed peaches to the prepared baking dish.
Sprinkle the dry cake mix evenly on top of the peaches, even out, and gently press down so the cake mix absorbs some of the juice.
Sprinkle remaining ¼ cup of brown sugar on top of the cake mix.
Evenly spread the thinly sliced pats of butter on top of the cake mix, making sure they are fairly close together and cover the entire top of the cobbler.
If desired, add another sprinkle cinnamon on top of the butter.
Bake the cobbler uncovered for 40-50 minutes in the preheated oven, checking for dry patches after about 10 minutes in the oven.
When it’s done the cobbler should be bubbling up and the peach filling should be nicely thickened.
Remove from oven and let cool.
Serve warm or room temperature. Add whipped cream or ice cream and enjoy!