Preheat oven to 425°F. Place a large baking sheet covered in foil on the lowest rack in the oven. This cobbler overflows a bit almost every time I make it. The baking pan will catch the drips and prevent a mess in the oven.
In a large bowl, combine the peaches, blueberries, lemon juice, sugar, cornstarch, and cinnamon. Stir to coat and then pour into a 9x13 baking dish. Bake for 10 minutes.
Make the Topping
While the filling is in the oven, whisk together the flour, brown sugar, baking powder, and salt. Toss the grated butter into the flour mixture. Stir in the boiling water, just until combined, leaving plenty of little lumps of butter.
Finish the Cobbler
Remove the filling from the oven and spoon the lumpy batter on top of it. (I like to use my smallest cookie scoop to do this.) Mix together the cinnamon and sugar and sprinkle it onto the cobbler topping.
Bake until the crust is golden and a toothpick inserted into the crust comes out clean, about 20-24 minutes. Let cool for 10 minutes. Serve warm with a scoop of vanilla ice cream, if desired.
Notes
Using a cheese-grater to "grate" cold butter is my favorite hassle free wait to cut in the butter. It takes just a moment and I think it's simpler than the more traditional methods. If you choose not to use the grater, simply cut the cold butter into small pieces and then blend it into the flour mixture, using a pastry blender or your fingertips.It is entirely up to you whether or not you choose to peel the peaches for this cobbler. I can hardly tell a difference and I doubt I'll ever again peel peaches for a dessert.