Start by crushing the Biscoff cookies into fine crumbs, either using a food processor or placing them in a zip-lock bag and crushing with a rolling pin. Set aside.
In a large mixing bowl, combine the heavy cream, mascarpone cheese, powdered sugar, and vanilla extract. Use an electric mixer to beat the mixture until soft peaks form—this should take about 3-5 minutes.
In a rectangular baking dish (9x13 inches works well), spread a thin layer of the cream mixture on the bottom. Next, dip a few Biscoff cookies in a shallow dish of milk for a few seconds until slightly softened, then layer them on top of the cream.
Add another layer of the cream mixture on top of the soaked cookies, followed by another layer of cookies. Repeat this process, creating at least three layers, ending with the cream mixture on the top layer.
Drizzle the Biscoff spread over the top layer and use a spatula or a small knife to create swirls. Cover the dish with plastic wrap and refrigerate for at least 4 hours, preferably overnight to let the flavors meld.
Before serving, sprinkle chocolate shavings or cocoa powder on top for an elegant touch. Slice into squares and serve chilled.
Notes
For best results, refrigerate overnight to let flavors meld.