Preheat your oven to 350°F (180°C). Line baking sheets with parchment paper or silicone baking mats.

In a large mixing bowl, beat the softened butter, light brown sugar, and white sugar together using a hand mixer or stand mixer on medium speed until the mixture is light and fluffy, about 2-3 minutes.

Add the egg and vanilla extract to the butter-sugar mixture, and continue to beat until well combined.

In a separate bowl, whisk together the plain flour, cocoa powder, salt, baking powder, and baking soda until well combined.

Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Be careful not to overmix to ensure the cookies remain tender.

Fold in the milk, white and dark chocolate chips until evenly distributed.

Shape the cookies into balls and freeze the cookies for at least 3 hours, to minimise spreading from baking.

Bake in the preheated oven for 15-18 minutes, or until the cookies are set around the edges but still slightly soft in the center. They will firm up as they cool.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Enjoy your freshly baked chocolate cookies with a glass of milk or your favourite beverage!