- Pre-heat oven to 180°C.
- Dissolve blue butterfly pea flower powder in hot milk and set aside to cool completely.
- Sift together 100g caster sugar, flour and baking powder into a large mixing bowl.
- In a separate mixing bowl, combine egg yolks, vegetable oil and cooled blue milk together.
- In a clean bowl, beat egg whites with cream of tartar until soft peaks. Gradually add in remaining caster sugar tablespoon by tablespoon, whisking well each time. Continue to beat the whites until you achieve glossy, stiff peaks.
- Combine flour and blue mixture, being careful not to overmix.
- Incorporate ⅓ of the egg whites into the mixture, and mix well to loosen the batter. Add in the remainder of the egg whites and gently fold through.
- Pour batter into an ungreased 25cm tube pan.
- Place on bottom rack of the oven, lower temperature to 160°C and bake for 1 hr.
- Turn off the heat and let the cake sit in the hot oven for another 30 mins.
- Remove from oven and allow to cool completely before turning out of the cake tin.
- Slice cake and serve.