Reduce the whole Oreos to fine crumbs with a rolling pin inside a sealed plastic bag, or pulse them in a food processor. The crumbs should look even, with no large cookie pieces that could weaken the crust.

Work the melted butter through the crumbs in a medium bowl, then firmly compact them across the bottom and sides of a 9-inch pie pan with the flat bottom of a measuring cup. The finished shell should feel tightly packed rather than loose or sandy.

Stir the half-and-half and marshmallow fluff continuously in a medium saucepan over low heat until smooth, then take the pan off the burner and nest it in an ice bath until completely cool. The mixture should thicken as it cools, and the pan and filling should no longer feel warm before you continue.

Blend both liqueurs into the cooled marshmallow mixture, adding one to two drops of green food coloring if desired. The filling should have an even color without streaks, whether you leave it pale or tint it green.

Whip the heavy cream in a large chilled bowl with a cold whisk until it holds stiff peaks. Lift the whisk to check that the peak stands upright and the cream still looks smooth rather than grainy.

Use a rubber spatula to gently fold the cooled marshmallow mixture into the whipped cream, lifting from the bottom and turning the mixture over itself. Stop when the color looks uniform and the filling remains fluffy, without pockets of plain cream.

Transfer the mousse to the cookie shell, scatter extra Oreo crumbs over it if desired, and freeze for 2 to 3 hours. The filling should feel firm enough to hold a wedge rather than slump when cut.