In a bowl, mix together both your light brown sugar and caster sugar with your melted hard margarine (or butter). 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 fully combined once more.
Add your flour, bicarbonate of soda and salt to your bowl and mix until 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). Be patient - it will come together, and it won't be dry!
Add about 160g of the chocolate chips and give them a good mix in so they are dispersed evenly.
Cover your mixing bowl with clingfilm and chill in the fridge for about 45 minutes - 1 hour (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 into balls (about the size of a ping pong ball). If you want to be specific like me, I weigh each ball so they are the same. Mine were 35g each. With you leftover chocolate chips place 1 or 2 in the top of the ball, this will ensure you see some on top when the cookie is cooked!
Place them on your trays. 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 12-13 minutes - they should have flattened out to look more like cookies and be a lovely golden colour.
Remove from the oven and leave to cool on the baking tray before moving them.
For the chocolate base, melt your chocolate in a large glass bowl over some boiling water. Once it's melted, allow to cool briefly.
Take each of your cooled cookies one at a time and carefully dip their bottoms into the chocolate so it's completely covered. Be careful when picking the cookies up not to get any on the top - I guess it doesn't matter if you do! Place each cookie melted chocolate up on a baking tray. Once you've filled a tray, place these in the fridge to completely set.
Once they are set, they're ready to eat! Enjoy!
You can keep them in the fridge or in an airtight box. Don't keep them in the sun, or the chocolate will melt!!