In a large bowl, whisk mix, egg, milk, oil, and vanilla extract. Let batter rest 5 min; it will continue to thicken.
Meanwhile, in a bowl, combine brown sugar, cinnamon and butter.
Scrape batter into a lined sheet pan. Spread evenly to fill corners. Gently tap the pan on the kitchen counter a few times to help even out the batter and release air bubbles.
Dollop cinnamon mixture across the top of the batter. Using a toothpick or butter knife, swirl to desired look.
Bake until centre springs back when pressed with a finger and a toothpick inserted in the centre comes out clean, 12 min. Let cool in pan 10 min.
