In a food processor or blender, puree the thawed raspberries.
Working over a saucepan, force the puree through a fine sieve to remove the seeds. Bring the sauce to a simmer.
Stir the cornstarch with the water in a small cup and then stir it into the raspberries. Stir in the lemon juice. Simmer the sauce unti it is thickened and smooth.
Remove from heat and stir in the Chambord if desired. Cool completely, then cover and refrigerate. This raspberry sauce can be made 2 days in advance if needed.
Line a bundt pan with plastic wrap, leaving a few inches of overhang. Put it in the freezer while preparing the semifreddo.
In a large mixing bowl, beat together the egg yolks and sugar until the mixture is thick and pale, then beat in the mascarpone, the vanilla, and the lemon zest.
Beat the egg whites until they form stiff peaks. Then fold them gently, but thoroughly into the mascarpone/egg mixture.
Sprinkle about a tablespoon of the grated chocolate evenly onto the bottom of the chilled bundt pan.
Spoon half the mascarpone mixture into the pan, sprinkle evenly with the remaining chocolate. Then spoon in the remaining mascarpone mixture and smooth it carefully
Fold the plastic wrap over the mixture and place the pan in the freezer for at least 8 hours or overnight. Unmold the semifreddo onto a serving platter. Garnish with fresh raspberries and drizzling some sauce over the semifreddo.
Slice to serve and drizzle with more raspberry sauce.