Preheat the oven to 350°F (175°C). Grease a 9 x 13-inch baking dish.
In a mixing bowl, combine the sliced peaches, blueberries, sugar, cornstarch, vanilla extract, lemon juice, and cinnamon. Gently stir until the fruit is well-coated. Pour the mixture into the prepared baking dish.
In another mixing bowl, whisk together the flour, baking powder, salt, and sugar for the biscuit topping.
Using a pastry cutter or fork, cut in the chilled butter until the mixture resembles coarse crumbs.
Pour in the buttermilk and gently stir until just combined. Do not overmix; it’s okay if there are small lumps.
Drop spoonfuls of the biscuit dough evenly over the fruit filling. It doesn’t need to cover the entire surface, as it will spread while baking.
Bake in the preheated oven for 35-40 minutes or until the biscuit topping is golden brown and the fruit is bubbling.
Remove from the oven and let it cool for a few minutes before serving.
Serve warm with a scoop of vanilla ice cream or whipped cream for an extra treat.