Preheat the oven to 350 F. Add the cubed butter to a 9 x 13 inch baking dish or pan. Place the dish in the oven for a few minutes until the butter is melted.
Meanwhile, in a medium bowl, combine the flour, granulated sugar, baking powder, and salt. Add the milk and vanilla extract and stir until smooth.
Pour the batter evenly over the melted butter in the pan (do not stir!). The melted butter will rise up over the batter.
Toss the peach slices and blueberries with the lemon juice and spread them evenly over the batter (again, do not stir).
Sprinkle the brown sugar evenly over the top and and bake 40-50 minutes until the top is golden brown. Allow the cobbler to cool a few minutes before serving.