Prepare the Dough: In a large mixing bowl, cream together the butter, dark brown sugar, light brown sugar, and granulated sugar until light and fluffy.
Add the egg, egg yolk, and vanilla extract, mixing until well combined.
Sift together the plain flour, cocoa powder, baking soda, and salt in another bowl. Gradually fold this dry mixture into the wet mixture until a dough forms.
Incorporate Chocolate: Melt 100g of milk chocolate and mix it into the dough. Then, fold in the dark and white chocolate chunks.
Cover the dough and refrigerate for at least 1 hour (up to 5 days for enhanced flavor).
Preheat the Oven: Set your oven to 190°C (170°C fan). Line a baking sheet with parchment paper.
Shape and Bake: Slice the chilled dough into 1.5 cm thick rounds or use a cookie scoop to portion out evenly sized cookies. Arrange on the baking sheet.
Bake for 8-9 minutes or until the edges are set but the centers remain soft.
Cool: Let the cookies cool on the baking sheet for a few minutes, then transfer them to a wire rack.
