To make the crust, process the oreos until fine. Add the melted butter and process again until they start clumping together. Press the mixture evenly onto the base of a 8 inch spring form pan. Refrigerate for 20 or so minutes for the crust to firm up.
Meanwhile, start with the pumpkin layer. Whisk together the puree, mascarpone and nutmeg. In a bowl, mix the gelatin and sugar and pour boiling cream and quickly dissolve the gelatin. Pour the gelatin mixture into the pumpkin bowl and mix thoroughly. Fold in the liquer and extract.
Pour the pumpkin cheese mixture into the pan and smooth the top. Freeze for 30 minutes or refrigerate for atleast an hour, to allow the gelatin to set sufficiently to make the next layer.
For the pecan layer, I find the easiest way to crush pecans is fold them in foil and beat them with a rolling pin. Also great for the angst! ;-)
Beat together the cheese, vanilla paste and two tablespoons of milk to loosen and fluff up the cheese. Prepare the gelatin mixture same as before with the remaining milk. Add this into the cheese mixture and whip to spread the gelatin evenly. Set aside. Whip up the cream into stiff peaks and gently fold into the cheese mixture. Fold in pecans.
Pour this layer on top of the setting pumpkin layer. Cover with wrap and refrigerate for atleast 3 more hours before decorating as desired, un-molding and serving.