In a large bowl, cream together the butter with the sugars for 2-3 mins using a stand mixer or electric whisk.
Add the egg and vanilla extract and beat again until combined.
In a separate bowl mix together the flour, cocoa powder, baking powder, bicarbonate of soda and salt.
Once the dry ingredients are mixed, sift these into the butter and sugar mixture. Mix until combined.
Add the chocolate chips and mix until evenly distributed.
Divide the cookie dough into 10 balls (approx. 75g each) and pop them into the fridge to chill for 2 hours.
When ready to bake, preheat the oven to 180°C fan and line a tray with baking paper or a silicone baking mat.
Place the balls of dough onto your prepared baking tray, making sure to leave space for the cookies to spread. You may need to bake the cookies in a few batches.
Bake in the oven for 10-12 mins. The cookies should feel soft to the touch when removing from the oven.
Leave the cookies on the tray for 10-15 minutes, then transfer them to a wire rack to cool.
Place the marshmallows into a microwaveable bowl and heat in the microwave for 10-15 secs, until the marshmallows begin to melt.
Stir the marshmallows and leave to cool slightly, you want them to be tacky when touched.
Take a small amount of the melted marshmallows between your fingertips and pull apart.
Continue to pull the marshmallow across the top of each cookie and repeat this process until you have covered the cookies enough to resemble spiderwebs.
Finally, place edible eyes on the cookies to decorate and enjoy!
