Place the marzipan together with the liqueur and lemon zest in a bowl and mash with a fork. Chop the almonds and mix in small quantities thoroughly throughout marzipan. Shape mixture into walnut-sized balls and chill.
Chop the chocolate and melt in a metal bowl over a hot water bath. Using a chocolate dipping fork, dip balls successively into the chocolate and place on parchment paper. Before the chocolate has set, decorate each ball with sliced almonds. Let dry before serving.