Grease and line a 20cm square tin, leaving a little overhang.
In a large saucepan, combine the sugar, golden syrup, cream, butter and Baileys. Place over medium heat and bring to a simmer, stirring regularly. `once the sugar is fully dissolved, increase the heat and bring to a rolling boil. Continue to boil, stirring occasionally, until the mixture reaches 116c on a sugar thermometer. At this stage the bubble will have changed from large and unruly to small and even.
Remove from the heat and pour ⅓ of the mixture over the dark chocolate. Add the white chocolate to the remailing ⅔ and stir to thoroughly combine.
Pour the white chocolate mixture into the prepared tin and drizzle over the dark chocolate mixture. Run a knife through the fudge to create a marbled effect and set aside to cool and set. Turn out and cut.