In a large bowl with an electric mixer or wire whisk, beat the egg whites until frothy. Add the cream of tartar, lemon juice, zest, and vanilla extract; continue to whip until the whites hold soft peaks. Sift the cake flour, sugar, and salt, together; gently fold the dry ingredients into the whites with a rubber spatula.
Take 2 (6-ounce) ramekins and put 10 raspberries in the bottom of each. Spoon the batter into the ramekins and set them in a bamboo steamer with a lid. Heat 2-inches of water in a wok over medium flame, set the steamer in the wok and steam the custard for 15 to 20 minutes.
Run a thin knife around edges of the ramekins and turn upside down onto plates. Garnish with raspberries, mint, and a dusting of confectioners' sugar.
VARIATION: To steam the custard at the same time as the Black Bass in Miso Soup entree, simply prepare the batter before assembling the steamers. Wrap the ramekins in plastic wrap (so they won't smell of fish) and put them in the top tier of the bamboo steamer. Steam everything together as directed in the recipe.