Boil some shredded purple cabbage for about 10–15 minutes. Remove the cabbage and continue cooking the liquid until it is reduced by half. Stir in a tiny pinch of baking powder. Next, stir in a tiny pinch of baking soda. Keep adding tiny amounts of baking soda until the liquid turns from purple to blue. (It’s important to only add as much as is needed to change the color, as too much baking soda will make it taste terrible.)

Once the liquid turns blue, add a drop of it to your icing. Start with a very tiny amount, as the flavor can be quite strong, depending on the cabbage.