For the quick birthday cake, break the chocolate into pieces and dissolve in a small saucepan in a water bath over low heat.
Coat the underside of half of the Leibniz butter cookies with a little chocolate and assemble with one cookie at a time.
Assemble the filled butter cookies with the remaining chocolate on a cake plate to form a cake, let the chocolate set.
To decorate, mix powdered sugar with enough lemon juice or egg white to make a thick, squeezable mixture.
Pour the icing into a parchment paper bag, cut off a small tip and label and decorate the birthday cake.
Top with colored sugar, pastry decorations and decorative flowers. Allow the icing to set.