PREHEAT oven to 350F. Spray a 12-cup muffin pan with cooking spray.
WHISK flour, sugar, cocoa, baking soda, baking powder and salt in a medium bowl.
WHISK egg, sour cream, oil, water and vanilla in a large bowl. Stir in flour mixture until just combined. Fill muffin cups a third full with batter.
BAKE until a skewer inserted in centre of a cupcake comes out clean, 16 to 17 min. Let cool in pan for 10 min, then transfer to a rack to cool completely.
PLACE coconut and food colouring in a resealable bag. Seal, then shake until evenly coloured. Transfer to a shallow dish.
FROSTING: Combine gelatin and water in a small bowl and stir until dissolved.
COMBINE egg whites, sugar, corn syrup, cream of tartar, salt and gelatin mixture in a large metal bowl. Set over a large pot with 1 in. of simmering water. Beat with an electric mixer on low until sugar dissolves, about 30 sec. Beat on medium-high until a stiff meringue forms, about 10 min. Remove from heat. Beat in vanilla.
FILL a piping bag fitted with a small plain tip with half of the frosting. Insert tip through the bottom of the cooled cupcake and pipe about 1 tbsp frosting into the centre just until sides of cupcake start to swell. Pipe a layer of frosting on sides and top of cupcake, and spread evenly. Place cupcake, frosting-side down, in coconut. Pipe frosting onto bottom and spread evenly. Roll cupcake to cover it in coconut, then place on a serving platter. Repeat with remaining cupcakes, refilling piping bag as needed.
Nutrition (per serving)
Kitchen Tip: Marshmallow frosting will set as it cools, so use immediately.