Preheat the oven to 325 degrees F. Coat a baking sheet with cooking spray. Whisk the egg whites, sugar, vanilla and salt in a large bowl until frothy. Fold in the coconut and cornflakes until combined. Scoop heaping tablespoonfuls of the coconut mixture, packing it tightly with your fingers, and arrange 1 to 2 inches apart on the prepared pan. Form each into a pyramid shape.
Bake until the macaroons are slightly firm and golden brown, 25 to 35 minutes. Let cool completely on the pan.
Meanwhile, combine the chocolate and coconut oil in a small microwave-safe bowl. Microwave until melted, about 2 minutes, stirring halfway through. Let cool slightly.
Dip the bottoms of the macaroons in the chocolate, letting the excess drip back into the bowl. Sprinkle the chocolate with nonpareils. Transfer the macaroons to a large plate or baking sheet and refrigerate until the chocolate is set, at least 30 minutes.