Vanilla party cake

Name: Vanilla party cake
Prep time: 1 h 15 min
Cook time: 40 min
Total time: 1 h 55 min
Cuisine: British
Ingredients: 225g unsalted butter , softened, plus extra for greasing; 350g caster sugar; 1 tbsp vanilla extract; 5 large egg whites; 325g plain flour; 25g cornflour; 1 ½ tbsp baking powder; 250ml buttermilk; 3 large egg whites; 240g caster sugar; 1 vanilla pod , halved lengthways and seeds scraped out; 360g unsalted butter , softened; 1 tsp vanilla extract; multi-coloured edible polka dots , to decorate