Butter and line a 22cm x 22cm tin, leaving a small overhang. Put the sugar, cream and Baileys in a large pan and, stirring slowly, bring it to a simmer. Make sure the sugar is dissolved (it will stop feeling grainy on the base of the pan), then turn the heat up to a rolling boil.
Adjust the heat until the mixture bubbles without getting too near the top of the pan. Keep bubbling, stirring occasionally, until a small amount of mixture dropped into a glass of cold water will form a soft ball that you can pick up on the end of a teaspoon.
At this stage, the bubbles will have gone from being large and unruly to smaller and more even. Stir in the chocolate and pour the mixture into the tin. Cool and cut.