Firstly make your base. In a food processor, blitz your biscuits into a crumb-like texture. If you don't have a food processor you can bash them with a rolling pin as well! You do want quite a fine consistency for this though.
Add your cocoa powder in and mix thoroughly or give another quick blitz.
Pour your melted butter into your cocoa crushed biscuits and mix together in a bowl.
Spoon your mixture into a loose bottom fluted tart tin (mine was 23cm across) and then using the back of a spoon and also your hands, press the mixture into the tin - both the base and up the sides. Try to get it as even and flat as you can. Using a (clean) jar or the bottom of a ramekin makes this job a doddle.
Chill this in the fridge for around 30 minutes - 1 hour. Whilst this is happening you can sort out your filling.
Slice your dark and milk chocolate finely with a sharp knife - this helps it melt quicker when you pour the cream onto it. Place it in a heat-proof bowl.
Also place your butter into the same bowl.
In a small saucepan, heat your cream and Baileys - do this gently so the cream is just about to reach boiling point (I stir mine a little so it doesn't stick to the bottom as it heats up).
Once heated, pour your Baileys and cream mixture on top of your chopped chocolate (in its bowl) and leave it for about 5 minutes without stirring. After 5 minutes mix it all together so that it is combined, smooth and fully melted.
Remove your tart tin from the fridge. Pour in your Baileys chocolate mixture and spread evenly with a spatula or palette knife.
Chill in the fridge for a few hours to allow the ganache to fully set (or overnight).
To serve, drizzle over your melted white chocolate and top with a little grated milk chocolate. Finish with Baileys truffles and gently press in a little to keep them still.
Slice up and enjoy. Keep refrigerated.