Preheat the oven to 200°C/fan 180°C/gas mark 6.
Place butter, sugar and Fate Low Protein All-Purpose Mix into a mixing bowl and rub with your fingertips until mixture resembles breadcrumbs. Then slowly add the water whilst mixing into a manageable dough.
Transfer the dough to a surface lightly dusted with Fate Low Protein All-Purpose Mix and knead until smooth.
Roll dough to a thickness of 0.5cm and using the biscuit cutter, cut into 6 biscuits, then place onto a lightly greased baking tray.
Bake for 20-25 mins, until golden in colour.
Remove from oven and cool on wire rack for 30 minutes.
On a microwaveable plate, place 1 of the biscuits, top with some marshmallows, and Vitabite then sandwich another biscuit on top.
Repeat step 7 with the remaining biscuits, marshmallows and Vitabite.
Cook each sandwich biscuit in the microwave for 30 seconds or until the marshmallows and Vitabite have melted.
Fate Low Protein All-Purpose Mix and Vitabite are foods for special medical purposes and must be used under medical supervision.