Grease a 9x5 inch loaf pan and line with two pieces of parchment paper so that the pan is covered and the edges of parchment overhang the sides.
Set a heatproof bowl over a pan of barely simmering water. Add the egg yolks, egg, and the BochaSweet, whisking continuously, until thickened, 5 to 7 minutes. Remove the bowl from over pan and cool the egg mixture to lukewarm, whisking frequently. Whisk in the extract.
In a large bowl, whip the cream with the powdered sweetener until stiff peaks form. Fold one large scoop of whipped cream into the egg mixture to lighten. Then gently fold egg mixture back into whipped cream until no streaks remain.
Stir in ¾ cup of the blackberries and ½ cup chopped hazelnuts. Spread the mixture in the prepared loaf pan and freeze until firm, at least 8 hours or overnight.
Run a spatula between pan and parchment paper. Invert the semifreddo onto serving platter and peel off parchment. Top with remaining blackberries and hazelnuts and cut into 1 inch slices.