Pre-Heat oven to Gas mark 4 (176°C) or 160°C for a fan assisted oven. Grease two large baking trays.
Beat the unsalted butter with an electric hand mixer until soft, add the sugar and beat until creamy, add the egg and Vanilla Extract and continue beating until well mixed.
Sieve the flour, bicarbonate of soda, salt, and Cocoa powder into the mixture and stir with a spoon until combined. Stir in all the chocolate chips until well distributed in the mixture
Chop the white and milk chocolate and place together in a pyrex bowl that fits snuggly over a sauce pan. Fill the sauce pan with water so that when the bowl is placed on the pan there is a gap. Bring the water to a simmer and place the bowl on the pan. Using this method melt the chocolate and then pour into the mixture. Stir the chocolate into the dough.
Shape the dough into walnut sized balls and place 10 on each tray squashing slightly and also leaving space between them as they will spread while cooking.
Bake for 10-15 minutes, 10 for soft and chewy cookies and 15 for harder crispy cookies.
Leave to cool on the baking tray, Once firm move to a wire rack.