Line a 9 x 5-in (23 x 13 cm) loaf pan with strips of parchment paper or with plastic wrap, letting excess parchment paper or plastic wrap run up over the sides of the pan. Set aside.
For the meringue
- In a large bowl, beat the egg whites until foamy, add the sugar and beat until stiff peaks form. Refrigerate until needed.
For the strawberry and raspberry layer
- In a large measuring cup, measure out ½ cup (125 ml) of the strawberries and ½ cup (125 ml) of the raspberries (set the remaining berries aside in a separate bowl). Add 2 tbsp. (30 ml) of the sugar and use a stick blender to puree until smooth (you can also use a stand blender or a food processor to make the puree). Strain the berry puree and discard the raspberry pits. Set aside.
- In a large bowl, beat the heavy cream with the remaining sugar (2 tbsp./30 ml) and vanilla until soft peaks form. Using a spatula, gently incorporate about a third of the meringue (leaving the rest of the meringue in the refrigerator) and the berry puree to the whipped cream. Transfer the berry mousse to the prepared pan and smooth it out. Sprinkle the saved diced strawberries and raspberries over the mousse. Freeze for at least 30 minutes.
For the dark chocolate layer
- Bring a pot of water to the boil and prepare the Dark Chocolate Fondueaccording to the packaging instructions. Remove the fondue can from the boiling water and pat dry. Open the can and pour the fondue into a small bowl. Set about ¼ cup (60 ml) fondue aside for service.
- In a large bowl, beat the heavy cream with the sugar until soft peaks form. Using a spatula, fold the dark chocolate fondue into the whipped cream and fold in the remaining meringue. Gently keep folding until no streaks of whipped cream or meringue remain (the color should be uniform).
- Spread the dark chocolate mousse over the frozen berry later and carefully smooth out the top. (depending on the size of the loaf pan you used, you may have some chocolate mousse leftover. If this is the case, simply transfer the extra chocolate mousse to a separate container and freeze to enjoy on its own). Freeze until the semifreddo is firm at least 6 hours or preferably overnight.
- Take the semifreddo out of the pan by pulling on the overhanging parchment paper or plastic wrap (run a sharp knife around the edge of the pan to loosen the semifreddo if needed). Invert the semifreddo over a serving plate, then peel the parchment paper or plastic wrap off. Garnish with additional berries, then drizzle the saved Dark Chocolate Fondue over the fruits (gently reheat the fondue before drizzling if it had time to solidify).
- Slice the semifreddo and serve immediately, or freeze until service.