Combine egg yolks and sugar in a bowl, whisking constantly over a double boiler until light and fluffy and all sugar is dissolved
Remove from heat and whisk in mascarpone cheese, being careful not to overmix
In a separate bowl, combine heavy cream, Biscoff butter, and vanilla extract
Whisk until stiff peaks form
Fold the whipped cream mixture into the yolk mixture until smooth
Brew coffee and stir in brown sugar and coffee liqueur if using
Quickly dip each Biscoff cookie into the coffee and layer them to fit the bottom of your baking dish
Spread a layer of mascarpone cream over the cookies
Dust with cocoa powder
Add another layer of coffee-dipped cookies
Drizzle melted Biscoff butter over the cookies
Add another layer of mascarpone cream
Finish with a drizzle of Biscoff butter or dusting of cocoa powder
Refrigerate for 24 hours before serving
