Line a loaf pan with plastic wrap and very lightly grease it.
In a small saucepan, combine water and sugar. Bring to a boil over medium-high heat. When sugar comes to a full boil, continue to boil for about 1 1/2 minutes.
While the sugar boils, beat egg whites to soft peaks in a large, clean bowl. When the sugar is ready, continue beating the eggs at a low speed and very slowly stream in the hot sugar. When all the sugar has been incorporated, turn up mixer to high and beat until meringue is glossy and has cooled down to almost room temperature, about 5 minutes.
In a large bowl, beat together the yogurt and mascarpone cheese. Split the Rodelle Vanilla Bean down the center and scrape out the seeds with the back of a knife. Add the vanilla bean seeds to the mascarpone mixture.
Fold the meringue into the mascarpone mixture.
Transfer mixture to prepared loaf pan, cover with plastic wrap and freeze for at least 6 hours, or overnight.
To serve, turn the semifreddo out of the pan and peel off plastic wrap.
