Heat oven to 350°F. Coat an 8x8-inch glass or ceramic square baking dish with cooking spray. Spread 2 ½ cups peeled and diced Honeycrisp apples evenly in the bottom of the dish.

Whisk 1 cup all-purpose flour, ¼ cup granulated sugar, 1 ¼ teaspoons baking powder, and ¾ teaspoon ground cinnamon together in a large bowl.

Whisk ½ cup whole milk, 1 large egg, 1 teaspoon vanilla extract, and ¼ teaspoon kosher salt together in a medium bowl. Pour into the flour mixture and stir together with a flexible spatula until smooth. Stir in 4 tablespoons melted unsalted butter until combined. Spread evenly over the apples in the baking dish.

Whisk 1 cup packed brown sugar, 4 teaspoons cornstarch, and ¼ teaspoon kosher salt together in a small saucepan. Whisk in ⅔ cup apple cider and ⅓ cup water. Bring to a boil over medium-high heat, whisking occasionally.

Add 4 tablespoons cut unsalted butter, reduce the heat to medium, and whisk until the butter is melted. Turn off the heat.

Immediately and gently ladle the hot sauce over the batter in the baking dish. Do not stir together; some small pieces of batter will float up.

Bake until the top of the cake is browned and a tester inserted into the center of the cake comes out clean, about 45 minutes. Let cool for at least 10 minutes before serving; the syrup will thicken slightly as it cools. Serve warm with vanilla ice cream or whipped cream.