Prepare the Cream Mixture: Beat the egg yolks and sugar together in a bowl until pale and creamy; add the mascarpone and vanilla extract, and mix until smooth; separately, whip the heavy cream to stiff peaks; gently fold the whipped cream into the mascarpone mixture until combined.
Make the Soaking Mixture: Dissolve the instant coffee and sugar in hot water; let it cool to room temperature.
Assemble the Tiramisu: Dip Biscoff biscuits quickly into the coffee mixture (don’t oversoak) and arrange them in a single layer at the bottom of your container; spread a layer of the mascarpone cream over the biscuits; repeat the layers (biscuits → cream) until you reach the top of the container; end with a layer of cream.
Decorate: Sprinkle crushed Biscoff biscuits on top as a garnish.
Chill: Cover and refrigerate for at least 4–6 hours (preferably overnight) to allow the flavors to meld.
Serve: Slice and serve chilled!