Arrange a large bamboo steamer or a large vegetable steamer over simmering water. Make sure it is large enough to hold a 10 inch tube pan.
In a large glass or metal mixing bowl, beat egg whites until foamy. Gradually add sugar, continuing to beat until stiff peaks form. In a medium bowl, beat together egg yolks, milk and vanilla. Quickly fold into whites until no streaks remain. Sift together flour and baking powder, then quickly fold into egg mixture. Pour batter into 10 inch tube pan.
Place the pan into the steamer. Steam for about 25 to 30 minutes, or until a toothpick inserted into the center comes out clean. Cool on a wire rack.