Grease the a square cake tin or baking pan with butter and chop up white chocolate into small pieces.
Measure out 125ml of Baileys and pour into a medium saucepan. Add the milk, butter and caster sugar and put the pan over a low heat. Stir occasionally, until the sugar has dissolved and the butter melted this will take about 10 minutes.
Turn up the heat to medium and let the mixture gently simmer while stirring constantly. You want the bubbles to be calmly breaking the surface for 10 minutes.
Remove the pan from the heat and stir in chopped-up chocolate and last 25ml of Baileys. Keep stirring until the chocolate has melted and you have a beautifully smooth consistency.
If the chocolate is being slow to melt, put the pan back over a low heat and stir until silky smooth.
Pour into the cake tin or baking pan and chill in the fridge for at least two hours, or until it has set completely
Remove from the fridge, cut into pieces and enjoy!