Dusted with edible glitter dust, these chocolate crackle biscuits are very glamorous. They are sandwiched with a marshmallow-fluff filling.
Heat the oven to 190°C/170°C fan/375°F/Gas 5. To make the biscuits, beat the butter in a mixing bowl until creamy. Add the caster sugar and beat for 5–10 minutes, or until the mixture is combined and very fluffy. Break the egg into the bowl and beat well until thoroughly combined.
Sift the flour, cocoa powder, bicarbonate of soda and salt into a separate bowl, then slowly add to the butter, sugar and egg mixture to make a stiff dough.
Divide the dough into 48 portions of 15g. Shape each into a neat ball and place on the prepared baking sheets, leaving plenty of space for expansion.
Tip the edible glitter or icing sugar into a saucer. Dip the base of a flat-bottomed glass into the saucer to coat, then press it down on each dough ball to flatten.
Bake the biscuits for 9–10 minutes, until just firm on top and dry along the cracks. Remove from the oven and leave on the baking sheets for 5–7 minutes to cool and firm up, then transfer to a wire rack to cool completely.
Meanwhile, make the marshmallow fluff. Put the sugar and syrup with 60ml of water into a small pan. Place over a low heat and stir frequently until the sugar has dissolved. Turn up the heat and cook until the syrup reaches 116°C on a sugar thermometer (don’t let it boil over).
Meanwhile, put the egg whites and cream of tartar into a mixing bowl or the bowl of a stand mixer and whisk slowly until foamy.
Once the sugar syrup is hot enough, steadily pour it onto the egg whites, whisking as fast as you can as you pour. Whisk for 8–10 minutes, until you have a very thick, fluffy and glossy mixture at room temperature. Finally, beat in the salt and vanilla bean paste.
Transfer the marshmallow fluff to a large disposable piping bag, snip the end and pipe a generous amount onto the underside of 24 biscuits. Place the remaining biscuits, undersides downwards, on top to make 24 filled cookies. Dust the cookies with a little more glitter dust and/or icing sugar, if you wish.