Method: Preheat the oven to 180C. Melt butter in a microwave oven or heat over boiling water in a bowl. Separate the egg yolks and the egg whites.Add the melted butter into milk, then sift cake flour into it and mix well. Add whisked egg yolks and vanilla extra into the mixture, mix thoroughly.Use an electrical mixer to beat egg whites for 3 minutes, then add cream of tartar and blend again. Pour sugar in the egg whites and blend until very stiff peaks form. Fold egg whites into the flour mixture gently with a rubber spatula just until all ingredients are incorporated. Do not stir or beat. For a better result, fold in egg whites with small amount at a time for 3 times.Pour batter into cup cake molds that are already lined with baking paper. Bake at 180C for 25 to 30 minutes. Check with a toothpick to see if the cakes are done. If the toothpick comes out of the cake clean, then they’re ready. Allow the cakes to cool for a minute or two in the pan, then transfer to a wire rack to cool completely.