Use a 24cm springform tin. Grease the base or line with baking paper. Preheat oven to 180 degrees C.
To make the cake, beat the egg whites until stiff. Using electric beaters, beat the butter, sugar, 4 egg yolks and orange zest in a medium bowl until the mixture is light and fluffy, about 5 minutes.
Sift flour and baking powder; stir into the butter mixture a tablespoon at a time. Fold egg white into mixture with a metal spoon.
Transfer mixture to the tin and smooth the top. Bake cake for 35–40 minutes until golden brown. Leave in the tin.
Stir ingredients for the glaze in a small saucepan over low heat until sugar dissolves. Prick hot cake a few times with a fork and drizzle with glaze. Stand tin on a wire rack and leave cake to cool completely.
Try this, too…
To make a Tipsy Lemon Cake, use lemon juice and zest and a lemon or almond-flavoured liqueur.