Put the almonds, icing sugar, zest and marmalade into a medium bowl and work together with a wooden spoon. It will look as though the mixture is too dry but keep working and the ingredients will come together.
Turn out onto a surface dusted lightly with icing sugar and knead the almond paste for 1-2 minutes until it forms a smooth ball. Wrap in clingfilm and freeze for about an hour to firm up.
Roll the paste out on a sugar-dusted surface or baking parchment to about 1 ½ cm thick and stamp out 12-14 almond or leaf shapes – or simple rounds. Use whatever cutters you have.
Roll out the fondant icing to about ½ cm thick and stamp out the same number of shapes.
Brush the top of each almond paste shape with a little extra marmalade and top each one with icing. Leave overnight on a wire rack to dry out and set before packing into air-tight containers. Eat within 1 week.
If you don’t have any cutters, simply roll out the almond paste to a rough square, brush with marmalade and lightly press the rolled out icing on top. Cut into bite-sized squares or diamond shapes with a sharp knife.
The calisson also freeze well for up to 1 month.