Mix together the cookie crumbs and melted butter and press down in a lightly buttered 9 inch (23 cm) pie dish. Place in the refrigerator while you make the Mousse.
BAILEYS MOUSSE
In a medium pot whisk together the sugar and flour, then slowly add the milk, whisking as you add with the heat on low / medium until the mixture has become very thick, stirring constantly. Cover with plastic and set aside to cool.
Meanwhile whip the cream until stiff peaks form.
Once cool, whisk briskly to remove any lumps that may have occurred. Fold in the Baileys, then fold in the whipped cream.
GELATINE MIXTURE
In a small pot mix the gelatine and whipping cream, let sit for 5 minutes. Then heat on medium / low and stir just until the gelatine dissolves, do not boil. Add a couple of tablespoons of the Baileys Mousse and stir to combine. Then fold into the remaining bowl of Baileys Mousse until combined.
Remove crumb base from the fridge and pour the Baileys Mousse on top. Cover lightly with plastic and refrigerate 8 hours or overnight.
CHANTILLY CREAM
Just before serving beat the whole cream and powdered sugar, and decorate the set Mousse Pie*. Drizzle with melted chocolate if desired. Enjoy!
*I covered the pie with 1/3 of the cream, then added a simple flower tip around the border.