Makes 14 cookies
Preheat the oven to 170°C fan.
Mix the sugars in a bowl, add the butter and beat. Add the egg and vanilla extract and beat again.
In a separate bowl, mix the flour, custard powder, salt, baking powder and soda and add to the wet ingredients. Work into a dough.
Roughly chop up the chocolate bars, keeping a bit to sprinkle over the cookies, and mix well.
Put 6 heaped tablespoons of the cookie batter onto a baking tray lined with baking paper – no need to push the batter down, but leave a good space between the cookies, as they will spread in the oven. Put some more chocolate pieces on top of the dough.
Bake in the preheated oven for 18 minutes, until golden around the edges and still very soft in the middle.
Leave the cookies to cool on the baking tray for a few minutes as they will be too soft to touch, then slide the baking paper with the cookies on top off the tray onto a wire rack and leave to cool completely.
Once cool, take the cookies off the baking paper and eat immediately or store in a tin for a week.