In a large bowl, combine 5 cups of peeled and sliced peaches with 3 cups of blueberries. Sprinkle in 3 tablespoons of granulated sugar, 2 tablespoons of cornstarch, and 1 teaspoon of vanilla extract. Gently stir until the fruit is well-coated and the sugar begins to dissolve. Let it sit for about 10 minutes.

In another bowl, whisk together 1¼ cups of all-purpose flour, ⅓ cup of granulated sugar, 1½ teaspoons of baking powder, ½ teaspoon of cinnamon, and ¼ teaspoon of salt. Cut 5 tablespoons of cold, cubed butter into the dry ingredients until it resembles coarse crumbs. Stir in ⅓ cup of buttermilk until just combined.

Lightly grease the interior of your slow cooker. Crumble half of the cobbler dough over the bottom, forming an even layer. Spoon the fruit mixture over the dough, evenly spreading it.

Cover the slow cooker and cook on high for 2½ to 3 hours, or on low for 4 to 5 hours, until the filling is bubbly and the dough is cooked through.

Once cooked, remove the lid and allow to cool for 20 to 30 minutes before serving.