Melt the dark chocolate bars in a bowl until perfectly smooth.

Temper the chocolate by pouring two-thirds onto a clean surface, spreading it until it cools, and then adding it back to the original bowl.

Mix the chocolate until it reaches 32 degrees Celsius (89.6 degrees Fahrenheit).

Pour the tempered chocolate into a chocolate bar mold, ensuring all sides are coated.

Tip out the excess chocolate and scrape the mold clean. Let the chocolate shells set.

In a saucepan, combine sugar, water, and corn syrup.

Heat the mixture to make a sugar syrup. While it's heating, separate an egg white into a bowl.

When the syrup is almost ready, begin whisking the egg white until foamy.

Once the sugar syrup reaches 115 degrees Celsius (239 degrees Fahrenheit), remove it from the heat.

Slowly and steadily stream the hot syrup into the egg whites while continuously whisking until a thick, glossy marshmallow fluff forms.

Transfer the marshmallow fluff to a piping bag and pipe it into the set chocolate shells.

Break Biscoff biscuits and press the pieces into the marshmallow fluff.

Top off the bars with the remaining tempered chocolate to seal them.

Let the chocolate bars set completely before demolding.