Preheat the oven to 180C (160Cfan) gas 4. Grease and line a shallow 20cm square tin with parchment paper.
Put the butter, marshmallows, Tate & Lyle sugar and Lyle’s Golden Syrup into a medium pan and heat gently until melted. Stir in the oats, when completely coated with the melted mixture, gently fold in the crispy cereal. Turn into the tin, level and press the mixture evenly and firmly with the back of a spoon. Bake for 25 minutes or until just golden around the edges.
Remove from the oven and cool for10 minutes then cut into 25 pieces whilst still warm. When completely cool, turn onto a cooling rack.
Mix the icing sugar with 2 tsp water until smooth. Drizzle over the flapjack and scatter the chocolate beans Allow to set then slide onto a board and cut into portions with a sharp knife.