Preheat your oven to 425 degrees and lightly grease a pie plate or an 8-inch square or round baking dish.

Peel and pit your peaches, slicing them into chunks about the same size as your raspberries. Place the peach chunks and raspberries into a medium bowl, and stir in the brown sugar, cornstarch, vanilla extract, and salt until combined and everything is coated. Pour this mixture into your prepared baking dish.

In a large bowl, stir together the flour, baking powder, salt, and sugar. Cut in the cold butter, using your hands or a pastry cutter to press the butter into the flour mixture until the butter is in small pea-sized pieces. Pour in the milk and vanilla extract and work the dough together with a fork until everything is well combined and there are no more dry pockets of flour. You should have a fairly soft, sticky dough here.

Dollop spoonfuls of the topping over the fruit filling until it's well covered with only a few spots of fruit peeking out. Generously sprinkle the demerara sugar over the top.

Bake for 25-30 minutes, or until the fruit is bubbling and the top looks golden brown and set all over. Enjoy warm with whipped cream or ice cream.