Grease and flour a 13x9-inch (33x23-cm) cake pan.
Preheat oven to 400°F(205°C).
To make cake, cream butter, vanilla sugar, half the unflavored sugar and salt in a large bowl until pale and fluffy.
Beat in 4 egg yolks one at a time.
Add orange peel and juice.
Beat egg whites until stiff; beat in remaining unflavored sugar.
Fold egg white mixture into creamed mixture.
Sift flour with baking powder.
Stir in almonds; fold into egg mixture.
Spread batter evenly in prepared cake pan.
Bake 10 minutes or until a wooden pick inserted in center comes out clean.
Cool slightly in pan then turn out onto a rack to cool completely.
Cut cooled cake horizontally into 2 layers.
Join layers together with orange jelly or marmalade.
To make frosting, combine powdered sugar, orange juice and Cointreau in a small bowl.
Pour frosting over cake.
Cut into slices when frosting has set.