Sadly, mascarpone cheese is highly prone to splitting. You'll know if it does as the contents of your bowl will look grainy, perhaps even reminiscent of cottage cheese. Mascarpone can curdle if it is whipped with other ingredients of a higher temperature, so make sure both the cheese and eggs are well chilled before you start. Older cheese is also more prone to splitting than fresh and similarly, low fat varieties are too. Always use quality, full-fat and fresh mascarpone for this recipe. Even then there is still a risk of splitting - this ingredient is a notoriously tricky beast. The best way to reduce the risk of curdling is to beat the mascarpone either by hand or using electric beaters on a low-speed setting. Beat in 15 second bursts until just smooth. At this point stop beating.