Grease and flour a 25-cm square cake tin. Whisk the eggs and castor sugar together until light and thick. Sieve the flour and salt. Fold into the mixture alternating with the butter. Add the orange zest.
Pour into prepared cake tin and bake for 20 to 30 minutes. Allow to cool in the tin for two to three minutes, then turn out onto a rack.
Once cooled, slice in half through the middle. Spread the apricot jam on one slice and sandwich together.
Knead the marzipan, then dust the surface with icing sugar and roll out to the same size as the cake.
Cover the cake with marzipan and smooth out. Refrigerate for one hour.
Use cutters to cut the cake into shapes of your choice, or cut into squares.
To make the glacé icing, slowly add the orange juice to the icing sugar, stirring to form a smooth, pouring consistency. In separate little bowls mix portions of the icing with various food colourings. Pour over the petits fours to coat and allow to set.
Decorate with shapes cut from the remaining marzipan. When dry, place in white paper cases.