Place a rack in middle of oven; preheat to 350°. Lightly coat an 8x8" metal baking pan with nonstick vegetable oil spray. Line with parchment paper, leaving overhang on all sides. Lightly coat parchment with nonstick spray.

Place 1¼ cups (156 g) all-purpose flour, 1 cup (packed; 200 g) light brown sugar, 1 tsp. salt, 1½ tsp. ground cinnamon, ½ tsp. baking powder, and ½ cup (1 stick) unsalted butter, melted, in a large bowl and mix with your hands, squeezing together as needed, until marble-size clumps form. Set streusel aside.

Toss 1 lb. Honeycrisp apples (about 2 large), peeled, cored, thinly sliced, slices halved crosswise, and 2 Tbsp. (16 g) all-purpose flour in a large bowl to coat.

Whisk 1 cup (200 g) granulated sugar, 1 tsp. salt, 1½ tsp. baking powder, ¼ tsp. baking soda, and remaining 1⅔ cups (208 g) all-purpose flour in a large bowl to combine.

Add ½ cup (1 stick) chilled unsalted butter, cut into pieces, and rub into dry ingredients with your fingers until no visible pieces remain. (Mixture should look like lightly moistened sand. Don’t worry about overworking it.)

Add 2 large eggs, ⅔ cup buttermilk, 2 Tbsp. vegetable oil, and 1 Tbsp. vanilla extract and stir until no pockets of dry flour remain, about 1 minute. Mix in about two thirds of apples.

Scrape batter into prepared pan and spread into an even layer; smooth surface. Scatter reserved streusel and remaining apples over, letting them organically intermix.

Bake cake until a tester inserted into the center comes out clean, 65–75 minutes. Transfer pan to a wire rack and let cake cool in pan.

Using parchment overhang, lift cake out of pan and transfer to a cutting board. Dust with powdered sugar and cut into 12 pieces if serving immediately.

Do Ahead: Cake can be made 5 days ahead. Store tightly wrapped at room temperature.