Preheat oven to 375 degrees. In a large bowl, combine flour, sugar, and salt. Add butter and cut with a pastry knife or fork until mixture resembles coarse meal. Add milk and mix until incorporated. Place dough between 2 pieces of plastic wrap and use a rolling pin to roll into an 11x15 inch rectangle. Press dough into the bottom of a 9x13 baking dish and set aside.
Peel and slice peaches into wedges. Add peach, maple syrup, sugars, cinnamon, and vanilla to a large bowl and stir to combine well. Mix in flour. Gently stir in blueberries. Spread filling evenly on top of crust.
In a large bowl, combine flour and sugars and mix well. Stir in cinnamon and salt. Add chilled butter and use a pastry cutter or fork to mash until fine crumbs form. Sprinkle topping over filling. Cover with foil and bake about 40 minutes or until crust begins to brown slightly. Remove and allow to cool slightly before serving.
Combine all ingredients in a saucepan and bring to a boil over medium heat; whisking continuously . Reduce heat and allow to simmer about 15 minutes. Drizzle sauce over cobbler-crisp while still hot (can also be chilled if preferred.) Serve cobbler with vanilla ice cream if desired.
Flavor tip: add a sprinkle of sea salt flakes on top just before serving.