Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a medium bowl, combine the blueberries, sliced peaches, 1/2 cup of the sugar, cornstarch, vanilla extract, and lemon juice. Stir gently to combine and allow to sit for about 10 minutes.
In another bowl, whisk together the flour, baking powder, baking soda, salt, and remaining 1/2 cup of sugar.
In a separate bowl, mix the melted butter and buttermilk until combined. Pour this mixture into the dry ingredients and stir just until moistened (do not overmix).
Pour the fruit mixture into the prepared baking dish, spreading it evenly. Drop spoonfuls of the dough mixture over the fruit, covering as much as possible.
Sprinkle the cinnamon over the top.
Bake in preheated oven for about 40-45 minutes, or until the top is golden brown and a toothpick inserted into the cobbler comes out clean.
Remove from oven and allow to cool slightly before serving.
Notes
Serve warm with a scoop of vanilla ice cream or whipped cream on top for added indulgence. Garnish with fresh mint leaves for a pop of color.