Gently warm the almond milk and vanilla bean paste in a medium saucepan and, just as the milk begins to simmer and steam rises from the surface, remove from the heat.
Using the coarse side of a box grater, grate the marzipan into the milk and stir intermittently until the marzipan melts, roughly 5–6 minutes.
If any pieces of marzipan are refusing to melt, briefly return the pan to the heat and continue to stir until they disappear.
Tip the xanthan gum into a small bowl and add 2 tablespoons of the warm almond milk mixture. Stir to combine until you have a thick, soft and uniform paste.
Return this paste to the saucepan and continue to stir until dissolved. Remove from the heat. Stir in the pectin and icing sugar and again, stir until smooth and uniform. Set the mixture aside to cool for 2 hours.
Pour into an ice-cream machine and churn to a soft set following the manufacturer’s instructions, or until the blade stops.
Spoon the soft ice cream into an airtight, freezerproof container and put in the freezer for at least 4 hours, or preferably overnight, until firm.
Remove from the freezer and allow the ice cream to soften for 5–10 minutes before scooping. Serve with pistachio nibs, if using.