Break the biscuits into a food processor and blitz until fine crumbs alternatively add them to a large food bag and bash with a rolling pin. With processor running add the cocoa powder, almond extract and melted butter. Until it begins to clump. If doing this by hand tip the bashed biscuit crumbs into a bowl and mix with a wooden spoon.
Tip the mixture into a 9" loose bottomed fluted flan tin. Using the back of a spoon or clean hands push the mixture up the sides of the tin, to form the sides and level the bottom. Refrigerate until set.
Meanwhile, prepare the filling by finely chopping the chocolate. Add this to a wide saucepan along with double cream, cocoa powder, caster sugar and amaretto. Stir continuously over a low heat until chocolate has melted and you have a smooth sauce.
Mix the cornflour into the milk in a small bowl, until no lumps remain then stir into the chocolate. Keep stirring the chocolate mixture until it has thickened about 8 - 10 minutes.. To test if it is thick enough, dip a wooden spoon in and run your finger through if the the line holds the mixture is thick enough.
Pour the filling into a clean bowl. Dampen a piece of parchment paper large enough to cover the filling. Place the parchment paper directly on top of the filling, this prevents a skin forming.
Allow to cool for 30 minutes.
When the filling has almost completely cooled. Retrieve the base from the fridge. Pour the filling into the tart case. Shake to level, bang gently on the counter to remove any air bubbles. Refrigerate for 6 - 24 hours until firmly set.
Remove from the fridge 15 minutes before serving. To release from the tin, set the tart on top of a bowl and pull by the sides to release. Set on a serving plate. Small slices will be adequate as it is very rich.