Pre-heat your oven to 160C Fan/350F/Gas Mark 4 and grease and line a 12" x 9" traybake tin
To make the Baileys cheesecake filling mix together the full fat cream cheese, caster sugar, egg and Baileys with a hand whisk or electric hand whisk until smooth
For the brownie mix start by melting the dark chocolate and butter in a glass bowl over simmering water (make sure the water does not touch the bowl), once melted, set aside to cool
In a bowl, whisk together the dark brown sugar and the eggs until frothy, add the Baileys and mix in
Slowly pour the melted chocolate and butter into the mixture, whilst constantly whisking
Add the plain flour and baking powder and mix in
Pour about two third of the brownie mixture into the traybake tin, then pour all of the cheesecake mixture on top, then pour the rest of the brownie mix on top in blobs
Using a knife, swirl the mixture around a bit, then bake it for 35 minutes, or until the cheesecake is set in the middle and slightly puffed up around the edges. A skewer inserted int the centre should have moist crumbs on it, but not be wet
Leave it to cool completely before cutting into squares
Store in the fridge and eat them within 3-4 days