Preheat oven to 425 degrees.

Add the apples, oil, and cinnamon to a 18- x 13- x 1-inch rimmed sheet pan; toss to coat. Bake in preheated oven until apples are tender and beginning to brown, 10 to 15minutes, stirring after 8 minutes.

In a blender, add the milk, eggs, salt, and vanilla. Blend until smooth, about 30 seconds. Whisk the flour and sugar in a bowl. Add flour mixture to milk mixture in 3 additions; blend until smooth after each addition.

Drizzle melted butter over apple mixture. Make sure butter completely coats bottom of pan. Pour batter evenly over apple mixture.

Bake at 425 degrees until puffy and golden, 10 to 15 minutes. Sprinkle evenly with powdered sugar. Cut into squares, and serve immediately.