Melt the chocolate. I melted it in 30-second intervals in the microwave, mixing between intervals. You can also melt it in a bain marie over low heat.
Line a 22x11cm loaf pan with baking paper. Cut it to the size of the pan and stick it in place with a bit of vegetable oil, spread with kitchen paper.
Place the egg yolks in a bowl to beat them and start beating on low speed just so that they don't form a dry skin on top.
Boil the water and sugar over medium heat. Let the sugar dissolve completely and then boil profusely. Once you reach that point (the syrup doesn't have to thicken), start beating the egg yolks on high speed and add the syrup down the side of the bowl.
Keep beating the egg yolks on high speed until the mix increases in volume and cools down to room temperature. Mix in the melted chocolate using a spatula and folding motions.
In a clean bowl, whip up the cream until foamy and glossy, not totally firm. Mix it with the egg yolk mix using folding motions. Also, add in half the praline in the same manner.
Pour the mix into the pan and freeze for at least 8 hours.
To serve, flip it over and place the rest of the praline on top. In the semifreddo is too solid, give it a few minutes before slicing.