Use full-fat buttermilk for a richer flavour in your cake. Adjust the amount of purple food colouring to achieve the desired vibrancy. Soften the butter before mixing for the best texture. Once baked and frosted, store the cake at room temperature in an airtight container for up to 3 days or refrigerate for longer freshness. You can also make this cake ahead and store it in the fridge for a few extra days if needed.