Prepare the Filling: Line a baking sheet with parchment paper and spoon out 12 tablespoons of white chocolate spread in small dollops. Freeze for at least 1 hour.

Mix Wet Ingredients: In a stand mixer, combine cold unsalted butter, brown sugar, and granulated sugar. Beat on low until clumpy, about 2-3 minutes.

Add Chocolate Chips: Gradually mix in white chocolate chips and dark chocolate chips until evenly distributed.

Combine Dry Ingredients: In a bowl, whisk together all-purpose flour, self-raising flour, black cocoa powder, crushed Oreo cookies, baking powder, and salt. Add to wet ingredients and mix until coarse breadcrumbs form.

Form the Dough: Add egg and yolk to the dough, mixing until thick. Weigh out 125g portions, flatten each, place frozen filling inside, seal edges, and roll in crushed Oreos.

Chill the Cookie Balls: Store cookie balls in an airtight container and freeze for at least 4 hours, preferably overnight.

Bake: Preheat the oven to 190°C (fan). Place frozen cookie balls on a parchment-lined baking sheet and bake for about 16 minutes, until edges are golden brown.

Cool and Serve: Allow cookies to cool on the baking sheet for about 5 minutes before serving warm.