Ombre Cake

Name: Ombre Cake
Prep time: 50 min
Cook time: 2 h 10 min
Total time: 2 h 10 min
Yield: 12 slices
Ingredients: 80 g unsalted butter; 250 g caster sugar; 4 egg whites; 150 g milk; 320 g plain flour; 2 tsp baking powder; 1 tsp natural vanilla extract; ½ tsp fine sea salt; food colouring paste; 180 g milk; ½ tsp fine sea salt; 200 g vegetable shortening; 1400 g icing sugar; 1 ½ Tbsp natural vanilla extract