Preheat the oven to 375℉.
In a large mixing bowl, combine the blueberries, peaches, sugar, lemon juice, salt, vanilla extract, and flour. Toss gently to coat the fruit evenly.
Pour the fruit mixture into a 10-inch cast iron skillet and spread it out evenly.
In a medium mixing bowl, whisk together the flour, sugar, baking powder, and salt.
Add the cold, cubed butter to the flour mixture. Use a pastry cutter or your fingers to cut the butter into the flour until the mixture resembles coarse crumbs.
Pour in the buttermilk and stir until just combined. The dough should be slightly sticky and clumpy.
Drop spoonfuls of the dough topping over the blueberry peach filling. Try to spread the dough evenly, but it’s okay if some fruit is still visible.
Place the skillet in the preheated 375℉ oven and bake for 35-40 minutes, or until the biscuit topping is golden brown and cooked through and the fruit is bubbly
Allow the cobbler to cool for about 10 minutes before serving. This will help the juices thicken slightly.
Serve warm, topped with vanilla ice cream or whipped cream if desired.