Arrange a rack in the middle of the oven and heat the oven to 350ºF. Coat an 8x8-inch square cake pan with cooking spray. Line the bottom and two opposite sides with a sheet of parchment paper that’s long enough to hang over the sides to create a sling.

Peel, core and dice 8 ounces crisp apples (about 1 ½ cups).

Melt 1 stick unsalted butter and in a medium nonstick frying pan over medium-low heat. Transfer to a large heatproof bowl. Add the apples to the frying pan (do not wipe out). Cook over medium heat until the apples start to soften, about 5 minutes. Transfer to a separate bowl.

Place 1 ½ cups all-purpose flour, 1 teaspoon ground cinnamon, and ¾ teaspoon baking powder in a medium bowl and whisk to combine.

Add 1 cup packed dark brown sugar to the bowl of melted butter and stir with a wooden spoon until combined. Add 1 large egg, 1 large egg yolk, 1 teaspoon vanilla extract, and ½ teaspoon kosher salt. Stir until smooth. Add the flour mixture and stir until just combined, being careful not to overmix; the batter will be thick and sticky. Add the apples and fold until just combined.

Transfer the batter to the pan and spread into an even layer. Use damp fingertips to gently dimple the surface of the batter (to mimic the texture of an apple fritter).

Bake until golden brown, and a tester inserted into the center comes out clean, 25 to 30 minutes. Let cool for 15 minutes. Grasping the excess parchment, lift the slab out of the pan, place on a wire rack, and let cool completely.

Place ½ cup powdered sugar and 2 teaspoons milk in a small bowl. Whisk until smooth.

Spread the glaze over the slab, making sure there is a thin layer over the entire top and letting a little pool in the crevices. Let sit until the glaze sets, 20 to 30 minutes. Cut into 12 bars.