Line an 8 inch square pan (preferably 2 to 3 inches deep) with parchment or plastic wrap, making sure paper or wrap extends about 3-inches up the sides over the pan.
Cut out two 8-inch squares of Genoise. Place one square of Genoise on the bottom the lined pan. Brush with simple syrup and freeze for about a half hour.
Remove cake from freezer, and panna cotta from the refrigerator, and pour the panna cotta on top of the frozen Genoise, smoothing it. Top with second square of genoise, and press down evenly. Brush second genoise with simple syrup, and pour the hazelnut ganache on top. Freeze for about 45 minutes to an hour.
Make the berry mousse while ganache is setting. Remove cake from the freezer and pour berry mousse on top of ganache and freeze for another hour, until frozen set.
Stir reserved berry puree or warm gently to loosen it. Remove cake from freezer and pour over the top of the mousse, Chill in the refrigerator until set. I prefer to freeze, then cut, to get nice slices, then bring to room temperature for serving, but since freezing kind of ruins the mirror, chilling in the fridge for the last set is fine. The cake will still be somewhat frozen when cutting, so you’ll get nice slices.
Lift cake from pan using plastic wrap or parchment overhang. Peel down paper or wrap then square off neatly by trimming raggedy edges. Slice cake into ½ -inch to 1-inch wide rectangles, depending on how many servings you need, or how thin or thick you’d like them. Top each slice with a little cocoa, berries, and maybe some chocolate curls, disks or spirals.