Preheat your oven to 200°C (or 180°C if you're using a fan oven). Line 2 large trays with baking paper or make sure you have non-stick cookie sheets to hand.

Mix the ground flax seeds with 1 tablespoon of cold water and set aside for 5 minutes. It will become thick and gelatinous.

In a large bowl, mix together the margarine, caster sugar and brown sugar. Add the flax seed mixture and stir again.

Add the flour and cocoa powder to the bowl. Mix it in until it starts to stick together, then use your hands to press it all together into a firm dough.

Add the digestive biscuit pieces, mini marshmallows and white chocolate chunks. Use your hands to gently knead them into the cookie dough.

Divide the dough into 16 equal sized pieces and roll each piece into a ball.

Flatten each of the balls slightly with your fingers, to around 1-inch thick, then place them onto the baking trays. Make sure to leave a bit of space between them to allow for spreading.

Bake for 7 minutes and let them cool completely before removing them from the tray. They will look slightly underdone but will firm up once they're cold.