Preheat oven to 180 C. Grease and line two 20.5 cm tins.
Cream butter and sugar together in a large bowl.
Add the eggs, one at a time, beating well after each addition. Add a spoonful of flour with the last egg to prevent curdling.
Stir in the vanilla. Sift in the remaining flour and salt and fold gently to combine.
Add enough milk to make a soft, dropping consistency.
Spoon the batter into the prepared tins and bake for 25-30 minutes. The cakes will be a lovely golden colour and coming away from the sides of the tin when ready.
Turn out onto a wire rack to coll completely.
To prepare the Irn Bru butter icing, combine the butter and sugar together in a food processor and beat well until light and fluffy. Add the Irn Bru until it reaches the desired consistency.
Sandwich the two cakes together with the icing and use the rest of cover. Decorate with 1 cm strips of coloured ready-to-roll icing in the Glasgow 2014 Commonwealth Games logo, if desired.