For best results, it’s important that the butter and mascarpone for this buttercream are both softened and at about the same temperature. An instant-read thermometer inserted into the center of each is an easy way to gauge when they’re ready to go. Our butter and mascarpone were at about 65°F (18°C). Try to handle the buttercream as minimally as possible; overworking it on your cake could cause the mascarpone to start breaking down. Some brands of mascarpone blend better into this buttercream than others. If yours is breaking instead of smoothly mixing in, you may need to try a different brand.