Preheat a fan forced oven to 180°C / 350°F. Spray two 8-inch baking tins with oil spray and line the bottom with baking paper.
To a large mixing bowl add the flour, cocoa powder, baking powder, bicarb soda, salt and malt powder. Use a whisk to combine. Add the brown sugar and mix to combine.
To a separate large mixing bowl add the buttermilk, oil, eggs and cookie butter and whisk to combine. Add the dry ingredients and mix to combine. The mixture will be thick. Add the vanilla extract and boiling water and mix until well combined.
Distribute the batter between the two baking tins, sprinkle each one with mini marshmallows and bake for 20-25 minutes. Allow to cool completely.