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Dutch-process cocoa powder gives the cookies their deep, inky color and just enough bitterness to balance the sweet vanilla filling. Golden syrup helps bind moisture in the dough, keeping the cookies crisp yet tender while adding a subtle caramel note. Rolling the dough to about an eighth of an inch keeps the cookies thin, delicate, and perfectly crisp for sandwiching. Cooking the butter removes excess water, so the filling stays firm at room temperature for that classic Oreo-style texture.