Set aside 1 cookie for later use. Process the remaining cookies and chocolate in a food processor until they form a crumb mixture. Add the butter and process until the mixture starts to clump together.
Press the mixture into a high-sided fluted tart pan, making a smooth base and sides with your hands or the back of a spoon. Put it into the refrigerator to chill and harden.
Melt the marshmallows in a saucepan over low gentle heat along with the milk. Once the milk starts to foam (not boil), take off the heat and keep stirring until the marshmallows blend into the milk to make a smooth mixture.
Pour the mixture out of the saucepan into a heatproof bowl, then whisk in the creme de menthe and creme de cacao. Leave until cool.
In a medium bowl, whisk the cream until it starts to hold soft peaks then, still whisking, add the cooled marshmallow mixture. This filling should be thick but still soft, not stiff or dry, so that it will eventually drop easily out of the bowl into the chilled pie crust.
When the marshmallow mixture and cream are combined, whisk in a few drops of food coloring, if desired.
Spread the filling into the chilled base, swirling it about with an icing spatula or silicone spatula to fill evenly. Put the pie in the refrigerator, covered, to chill overnight or for a minimum of 4 hours until firm.
Crush the remaining cookie and sprinkle it over the top of the pie before serving.
Make Ahead Note:
The pie can be made 1 to 2 days ahead. When chilled and firm, tent with aluminum foil (try not to touch surface with foil as it will leave marks) and store in the refrigerator. Decorate just before serving. The pie will keep 3 to 4 days total.
The pie can be frozen for up to 3 months. Open-freeze undecorated pie just until solid, then wrap the pie (still in its tart pan) in a double layer of plastic wrap and a layer of foil. To thaw, unwrap the pie and tent with foil (try not to touch surface with foil as it will leave marks), then thaw overnight in the refrigerator. Decorate before serving.
From Nigella Kitchen by Nigella Lawson. Copyright (c) 2010 Nigella Lawson. Published by Hyperion. All Rights Reserved.