Preheat your oven to 190c/170fan and line a 8"/20cm round tin with parchment paper, and leave to the side for now.
Whisk/mix the plain flour, bicarbonate, salt and cornflour together so its evenly distributed – leave to the side.
In a larger bowl, melt the butter. Add in the two sugars, and whisk for about 2 minutes so the sugar starts to dissolve and the mixture is smooth. Add in the egg and the vanilla, and whisk again briefly until smooth.
Add in the dry Ingredients, and the chocolate chips (apart from a small handful) and mix with a spatula or a mixer until a thick cookie dough is formed.
Press the mixture into the bottom of the tin and press in the handful of chocolate chips into the top for decoration.
Bake the cookie in the oven for 25-30 minutes, until the top of the cookie looks ‘dry’ and it's not wobbling!
Leave the cookie to cool for about 10 minutes, then leave to cool fully on a wire rack.
Once the cookie has cooled, beat the unsalted butter for a minute or two to loosen and soften.
Add the icing sugar and vanilla extract and beat again until smooth.
Pipe around the edge of the cookie however you want, and add any to the middle if you want.
I then used some shopbought edible happy birthday letters, and some sprinkles!
This cookie will be best eaten on the day of baking or the day after, but will last 4-5 days.
You can use any chocolate chip or chopped chocolate you want in the cookie - stick to a minimum of 200g, and a maximum of 400g.
If baking into 9"/23cm tin for a thinner but larger cookie, bake for 20-25 minutes.
If you can't access Cornflour - use 25g more flour, but the texture will change.
You can make the cookie chocolate themed by using 200g plain flour, and 50g cocoa powder.