Measure 12 tbsp of white chocolate spread and place on a baking sheet lined with parchment paper. Freeze until solid, about 1-2 hours.

Preheat the oven to 212°F (100°C). Spread the milk powder evenly on a baking sheet lined with parchment paper. Bake for 10-15 minutes, stirring occasionally, until the milk powder turns a light golden brown. Remove from the oven and let it cool completely.

In a saucepan over medium heat, melt the butter. Continue cooking, stirring frequently, until the butter foams and then turns a rich brown color with a nutty aroma. This should take about 5-7 minutes. Pour the browned butter into a large mixing bowl and let it cool slightly.

Add the toasted milk powder, caster sugar, and light brown sugar to the browned butter. Cream together until well combined. Add the egg yolks one at a time, beating well after each addition. Mix in the semi-skimmed milk until well combined.

Divide the wet ingredients mixture evenly into two separate bowls.

In one bowl, sift together the dark chocolate powder, plain flour, baking powder, bicarbonate soda, and cornstarch. Gradually add the sifted dry ingredients to the wet ingredients, mixing until just combined. Fold in the chopped white chocolate.

In the other bowl, sift together the plain flour, baking powder, bicarbonate soda, and cornstarch. Gradually add the sifted dry ingredients to the wet ingredients, mixing until just combined. Fold in the chopped milk chocolate.

Using a cookie scoop or spoon, alternate scooping the dark chocolate dough and toasted milk dough to form a marble effect. Place a frozen piece of white chocolate spread in the center of each cookie ball. Gently roll the dough to enclose the white chocolate spread and create a ball.

Place the cookie balls on a baking sheet lined with parchment paper. Refrigerate overnight to allow the flavors to meld and the dough to firm up.

Preheat your oven to 180°C (350°F). Arrange the chilled cookie dough balls on a baking sheet, leaving space between them for spreading. Bake for 12-15 minutes or until the edges are set and the centers are still slightly soft. Allow the cookies to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.