In a bowl, mix together both your light brown sugar and caster sugar with your melted hard margarine. Mix until it all comes together and is fully combined.
Add your egg and vanilla extract to the bowl and continue to mix (with a wooden spoon) until combined once more.
In a separate bowl, add your gluten free flour, bicarbonate of soda, salt and cocoa powder (I always sieve coca powder) then briefly mix.
Add your flour mix to your original bowl and mix till combined. Make sure your mixing bowl is big as this is quite a lot of flour. It can be hard work and takes time to really combine it all but it'll form a perfect cookie dough (you could use a stand mixer but I prefer to do cookie dough by hand).
Add the chocolate chips and give them a good mix in so they are dispersed evenly. I often just buy bars of dairy free chocolate and break them up to make my own chips.
Cover your mixing bowl with clingfilm and chill in the fridge for about 45 minutes (you can make the cookie dough ahead of time also and keep it in the fridge until you need it).
Heat your oven to 170C and prepare a couple of baking trays with good quality, non-stick baking paper.
Remove your cookie dough from the fridge and roll your dough into balls (about the size of a golf ball) and place them on your trays. This time I actually weighed my balls to ensure they were even. Each was about 60g. The cookies will spread a little so leave some gaps between them - I usually cook mine in a couple of batches.
Pop into the preheated oven for 15 minutes - they should have flattened out to look more like cookies.
Remove from the oven and leave to cool on the baking tray before moving them. They'll be very soft at first so allow to cool.