Preheat the oven to 180°C and line a 12-hole cupcake tray with cupcake cases.
Whisk together the plain flour, caster sugar, baking powder and salt in a large bowl. Beat in the unsalted butter until the mixture resembles breadcrumbs.
In a small jug, whisk together the egg, milk and vanilla. Slowly pour into the flour mix with the whisk on a low speed. Continue to beat until the mixture is smooth.
Fill the cupcake cases until they are ⅔ Bake for 12-14 minutes, or until golden brown and a skewer inserted comes out clean. Cool completely on a wire rack.
For the buttercream, sift the icing sugar into a large bowl and rub in the butter until the mixture resembles breadcrumbs. Add the milk and vanilla then beat together for 5-6 minutes, using an electric whisk, until the mixture is smooth.
Pour the popcorn onto a flat plate. Place a round 1.5cm nozzle into a piping bag and full with buttercream. Pipe large blobs of icing onto the surface of the cupcakes, then roll in the popcorn so that the buttercream is completely covered and resembles a sheep’s wool