Melt the Galaxy® chocolate in a microwave-safe bowl in microwave, cooking in increments of 30 seconds and stirring frequently.
Crush biscuits and Maltesers® in a separate bowl. Add marshmallows if using then pour over the melted chocolate and stir together to coat (can be popped in microwave for a few seconds to help melt the chocolate on the biscuits).
Evenly spread mix on greaseproof paper lined tray, score the top into desired serving size, pop in fridge to cool and harden (about an hour). Break into pieces and enjoy :-)