Preheat your oven to 190ºC/170ºC fan and line a 22cm heart shaped cake tin! 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 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 pink/red chocolates 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 in the tin for about 10 minutes, then leave to cool fully on a wire rack. But be careful removing the cookie from the tin whilst still hot!
Once the cookie has cooled, beat the unsalted butter for a minute or two to loosen and soften.
Add the icing sugar, vanilla extract and pink food colouring 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 sprinkles and more M&Ms to decorate!