Place the peaches in a large mixing bowl. Add the sugar and cornstarch and then toss to coat. Let sit to macerate while you prepare the topping.
In another mixing bowl, combine the flour, sugar, baking powder, ground cinnamon, and salt. Add the pieces of cold butter and cut the butter into the dry ingredients using a fork until it looks like sand. Stir in the milk.
Spray the slow cooker with non-stick cooking spray. Add the peaches to the crock and spread them into an even layer. Spoon dollops of the topping mixture over the peaches so they are almost completely covered.
Place a double layer of paper towels over the top of the slow cooker. Pull the towels taut and then put the lid in place. (The towels should overhang the edge of the crock and they will capture some of the excess moisture that is created during cooking.)
Cook for 4-5 hours on LOW or 3-4 hours on HIGH.
Remove the lid and towels from the cobbler. To serve, spoon portions into servings bowls and top with vanilla ice cream or whipped cream if desired.
Notes
The topping on this cobbler is more spongy and will be slightly wet since it is steamed instead of oven-baked like a standard cobbler.
If you can't find white peaches, yellow peaches can be substituted instead. You can peel the peaches or leave the skins on based on your preference.