Cut a circle of parchment paper to fit the bottom of a 9-inch springform pan.  As an option you can also line the inside of the pan with a strip of parchment paper.  Set aside.

Put the chocolate wafers into a food processor and pulse until they are very fine. (Optionally you can place the wafers in a sealable bag and pound them with a rolling pin then pour them into a bowl.)

Add the butter and the sugar to the food processor; then pulse the food processor (or stir them) until well combined. Press mixture firmly onto the bottom and up the side of the prepared springform pan.  Transfer the pan to the freezer for 20 minutes.

Chop the chocolate into pieces and add to the top of a double boiler which is set over barely simmering water and allow to melt undisturbed.  Remove from the heat and set aside to cool.

Put the cream cheese and the sugar in the bowl of an electric mixer.  If you have a stand mixer, use the paddle attachment. On medium speed, beat the mixture until smooth, about 4 minutes.  Add the vanilla extract, the Irish cream liqueur and the cooled chocolate, beat together until fully incorporated.

In a separate bowl lightly whip the cream to the soft-peak stage (until it has thickened and increased in volume slightly and will just hold its shape if you lift the whisk out of the cream).  Gently fold the whipped cream into the chocolate mixture.  

Remove the pan with the crust from the refrigerator or freezer and spoon the cheesecake mixture into the chocolate crust and smoothing the top. Drape a sheet of plastic wrap lightly over the cake and refrigerate until firm. About 2 hours.

Run a thin sharp knife around the inside of the pan between the crust and the metal, flip the metal side ring latch loose and lift off the side ring.  Slide the knife under the bottom crust to loosen from the metal pan bottom and transfer the cake to a serving plate.  

Optional:  Garnish the cake with chocolate curls or dust lightly with cocoa powder.

For each slice, use a thin sharp knife dipped in hot water then wiped dry.