Heat the oven to 180ºC. Grease baking cups with butter and put in 12 holes of a muffin tin or just put in a baking tray.
Sift together the flour and baking powder with salt in a separate bowl. Pour water to the egg yolks. Add vanilla and sugar and beat well until the sugar is dissolved and well blended.
Pour in the sifted dry ingredients and mix well to combine. Add the vegetable oil and stir well to have a good final mix.
Beat egg whites and add cream of tartar. Beat well until soft peaks are formed. Gradually add the sugar, beating well after each addition. Continue beating until egg whites are very stiff.
Fold a third of the egg whites into the batter to loosen it. Add the rest of the egg whites another third at a time. Spoon 3 tablespoon of the mixture into the baking cups. Bake for about 15 minutes, until golden.
Cool in the tin for 5 minutes then place the cups in a tray with baking paper upside down for 15 minutes to cool completely.
For the topping, spread gently the butter or margarine and roll the top with sugar or sprinkle sugar over the top and serve.