Preheat the oven to 350° F. In a large bowl, toss together peaches, blackberries, brown sugar, 1 tablespoon plus 1 teaspoon (use 2 tsp for a thicker filling) cornstarch, and vanilla. Spread the fruit out into a 10-12 inch baking dish or cast iron skillet. To make the topping, combine the oats, flour, brown sugar, 1/2 teaspoon cinnamon, and salt in a medium bowl. Add 8 tablespoons butter and cut the butter into the dry ingredients using a fork or pastry cutter. Sprinkle over the fruit. Cover with foil and bake 30 minutes. 4, Meanwhile, add 4 tablespoons butter to a small pot and set over medium heat. Allow the butter to brown lightly until it smells toasted, about 2-3 minutes. Remove from the heat and whisk in the maple syrup, 1/2 teaspoon cinnamon, and a pinch of salt. Remove the crisp from the oven, remove the foil, and drizzle the maple butter over the crisp. Bake, uncovered another 25 to 30 minutes. Let cool slightly, then serve with ice cream.