Unlike most types of cake, a sponge cake doesn't get its rise from a leavening agent such as baking soda or baking powder. It's the air that's been whipped into the separated eggs that makes a sponge cake so light and fluffy. For this reason, it's important to be patient when whipping the egg yolks and whites. Rushing through this task may result in a cake that is flat with a heavy grain.
Separate the egg yolks and whites by cracking them into a separating tool or into your cupped hand. If using your hand, open your fingers slightly and let the whites drip through them into a clean bowl. Place the yolks into a large mixing bowl.
Cover each bowl with plastic film and let the eggs sit until they come to room temperature -- approximately 30 minutes.
Add 1/2 cup of sugar to the egg yolks and beat on high with an electric mixer for approximately five minutes or until the yolks are thick and lemon coloured. Add the vanilla extract, milk and zest to the beaten yolks. Beat for a few more seconds to incorporate the ingredients.
Place the flour, salt and 1/4 c. of the sugar into a sifter. Sift the mixture over the beaten yolks. Do not stir.
Spoon the cream of tarter into the egg whites and beat with clean beaters until the whites form soft peaks. Gradually add the last 1/4 c. of sugar and continue beating until the mixture forms stiff, shiny peaks.
Fold the egg whites into the yolk and flour mixture with a rubber spatula. Work gently, taking care not to deflate the air in the egg yolks or whites. Do not over mix, fold just long enough to evenly incorporate the whites into the batter.
Pour the batter into an ungreased pan. Use a spatula to spread it out in an even layer. Bake in a preheated 350-degree Fahrenheit oven for approximately 50 minutes or until a toothpick placed in the centre of the cake emerges clean.
Invert the cake pan over a bowl or jar and cool for at least 60 minutes. When the pan is cool to the touch, disengage the cake from the pan by sliding a sharp, thin knife around the outside and inside edges of the pan. Invert the cake again and place on a serving platter.
Chilled eggs separate more cleanly than warm ones, keep your eggs in the refrigerator until you are ready to use them. Using a two-piece tube pan with a removable bottom may make unmoulding your sponge cake easier. Serve sponge cake with whipped cream and fresh fruit or with a light dusting of icing sugar.
Wash your hands frequently and thoroughly when working with raw eggs. Prevent cross-contamination by washing utensils with hot, soapy water after each use.