Put the custard and chocolate in a saucepan. Gently heat, stirring, until the chocolate has completely melted into the custard. Cover the surface with cling film to stop the custard forming a skin, then cool.
Sit the brownies in a trifle bowl and mix together the coffee with the 1 tbsp Irish cream liqueur. Drizzle all over the brownies. Use a rolling pin or saucepan to gently bash the bag of Maltesers a few times to crush a little, then sprinkle about three-quarters over the brownies. Spoon the cooled chocolate custard all over the top, then cover and chill.
Make the final layer by combining the cream and 100ml Irish cream liqueur in a bowl. Sift over the icing sugar, then whip until soft peaks form. Cover and chill until you’re ready to serve.
To serve, give the cream a quick mix, then spoon on top of the chocolate custard. Scatter over the last few crushed Maltesers to decorate.