Instructions

1. Heat the oven to 375°F. Coat an 8x8-inch or 2- to 2 1/2-quart baking dish with butter.
2. Microwave 1 stick cut-up unsalted butter in a medium microwave-safe bowl until just melted, 40 to 50 seconds. (Alternatively, melt the butter in a small saucepan over medium heat and pour into a medium bowl.) Let cool for 5 minutes.
3. Stir in 1 cup all-purpose flour, 1/2 cup packed light brown sugar, 2 teaspoons vanilla extract, 1 teaspoon baking powder, 1 teaspoon ground cinnamon, and 1/4 teaspoon kosher salt with a fork until evenly moistened and a variety of crumb sizes form. The mixture will be thick and slightly pasty (it will firm up as it chills). Refrigerate for at least 10 minutes.
4. Peel, core, and cut 2 pounds sweet-tart apples into 1/4-inch-wide slices and place in a large bowl. Add 1/4 cup packed light brown sugar, 2 tablespoons cornstarch, 1 tablespoon lemon juice, 1 teaspoon ground cinnamon, and 1/2 teaspoon kosher salt. Toss to coat.
5. Transfer the filling to the baking dish and spread into an even layer. Break the crumble topping into large clumps with your hands and scatter over the filling in an even layer.
6. Bake until the juices are bubbling around the edges and the topping is golden-brown and firm to the touch, 40 to 45 minutes. Check at 30 minutes — if the topping is browned, tent with aluminum foil for the remainder of time. Let cool for at least 15 minutes before serving with vanilla ice cream if desired.