Roast the peaches: Heat the oven to 400°F (200°C). Butter a 2-quart baking dish. Melt the 2 tablespoons of butter in a wide skillet over medium-high heat. Add the peach wedges and 1/4 cup sugar and cook 5 minutes, stirring, until just softened and golden at the edges. Stir in the vanilla, five-spice and lemon juice, then the cornstarch. Tip into the prepared dish.
Rub the topping: Whisk the flour and salt in a large bowl. Rub in the cold butter with your fingertips until it looks like fine breadcrumbs, then stir through the 3 tablespoons of sugar.
Make the dough: Make a well in the center, pour in most of the 2/3 cup of milk, and stir just until a soft dough forms, adding the last splash only if needed. Do not overwork it.
Top the fruit: Drop rough spoonfuls of dough over the hot peaches, leaving gaps so the fruit shows through. Brush the dough with a little extra milk and scatter over the demerara sugar.
Bake: Bake 25 to 30 minutes, until the topping is risen and golden and the filling bubbles at the edges.
Serve: Rest 5 minutes, then serve warm with vanilla ice cream or cream.
