Grease an 8-1/2-inch by 4-1/2-inch loaf pan with cooking spray and line with plastic wrap.
In a small saucepan over medium heat, combine the granulated sugar and water. Bring to a boil for 2 minutes. The sugar should be dissolved, and the mixture should have thickened into a syrup.
In a large bowl, using an electric mixer, beat the egg whites until they form stiff peaks. Gradually pour the hot syrup into the egg whites while the mixer is running. Beat until cool, about 2 minutes.
In a medium bowl, whisk together the heavy cream and vanilla seeds. Fold into the egg whites in 3 additions. Divide into 3 bowls with one batch a little smaller than the others.
In a blender, puree the strawberries, raspberries and confectioners’ sugar. Strain through a fine mesh sieve into the smaller batch of cream. Pour into the prepared loaf pan and freeze until firm, about 45 minutes to 1 hour.
Pour a layer of plain vanilla cream into the loaf pan and freeze until firm, about 45 minutes to 1 hour.
In a small bowl, melt the chocolate in 30-second blasts in the microwave. Allow it to cool, but still be runny before stirring it into the remaining cream. Pour it in the loaf pan and freeze at least 3-4 hours. Before serving, let the semifreddo sit at room temperature for 10 minutes and then slice.