Crush the Biscoff cookies into fine crumbs. Melt the butter and mix with the cookie crumbs, then press firmly into the tart pan.

Heat the heavy cream in a saucepan until simmering, then pour it over the chocolate in a bowl, letting it sit for 5 minutes.

Stir the chocolate and cream mixture until smooth, then add the cookie butter and mix until incorporated.

Pour the mixture into the prepared crust, smoothing the top with a spatula.

Cover with plastic wrap and refrigerate for at least 4 hours, or overnight for best results.

Slice and serve chilled; add whipped cream or ice cream if desired.