For the Crumble Topping

• 8 tablespoons (1 stick) unsalted butter, cut into 8 pieces, plus more for the baking dish
• 1 cup all-purpose flour
• 1/2 cup packed light brown sugar
• 2 teaspoons vanilla extract
• 1 teaspoon baking powder
• 1 teaspoon ground cinnamon
• 1/4 teaspoon kosher salt

For the Apple Filling

• 2 pounds sweet-tart baking apples (4 to 5 large), such as Granny Smith, Honeycrisp, or Pink Lady
• 1/4 cup packed light brown sugar
• 2 tablespoons cornstarch
• 1 tablespoon freshly squeezed lemon juice (from 1/2 medium lemon)
• 1 teaspoon ground cinnamon
• 1/2 teaspoon kosher salt
• vanilla ice cream, for serving (optional)

STEP 1

Heat the oven to 375 degrees F. Coat an 8-by-8-inch or 2- to 2 1/2-quart baking dish with butter.

STEP 2

To make the crumble topping: Microwave 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.

STEP 3

Stir in all-purpose flour, packed light brown sugar, vanilla extract, baking powder, ground cinnamon, and 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.

STEP 4

To make the apple filling: Peel, core, and cut sweet-tart apples into 1/4-inch-wide slices and place in a large bowl. Add packed light brown sugar, cornstarch, lemon juice, ground cinnamon, and kosher salt. Toss to coat.

STEP 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.

STEP 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.