Super Soft & Fluffy Butter Sponge Cake 烫面牛油蛋糕

Ingredients  用料(A)35g butter 用料牛油35g milk 牛奶10g granulated sugar 糖2 large, egg yolks, cold 蛋黄 (冷)1/2 tsp vanilla extract 香草精35g cake flour 低筋面粉a pinch of sea salt 一小撮海盐(B)2 large, egg whites, cold 蛋白 (冷)1/4 tsp cream of tartar or 1/2 tsp distilled vinegar 塔塔粉 / 醋35g granulated sugar, optional 糖Preheated oven 150ºC and hot water bath, steam bake with both top and bottom heat without fan assisted