Preheat the oven to 375 degrees F/190 degrees C. To make the biscuit, sift flour, cocoa powder, and potato starch using a flour sifter and set aside. In a bowl, beat egg whites with an electric mixer until they are mounted.

Gradually add granulated sugar and continue to whisk until stiff and glossy. Add egg yolks and gently mix with a rubber spatula, working in the same direction. Then add the dry mixture of flour, cocoa, and starch and gently combine with the spatula. Finally, add melted butter and mix again.

Pour the biscuit dough on a baking sheet covered with parchment paper or a non-spill baking sheet and even the surface with a bent spatula. Make sure to form a rectangle of 12 x 11 inches/30 x 27 cm. Bake biscuit for 10 minutes. Take it from the oven and let cool.  

To make chocolate mousse, whisk 2/3 cup (150 g) whipping cream to get the whipped cream and refrigerate. Melt chocolate in a bain-marie/water bath or microwave until the chocolate reaches 122 degrees F/50 degrees C. In a separate bowl, soak gelatin in cold water.

Place 3 tablespoons (45 g) of heavy cream and milk in a small saucepan and bring to a boil. In another bowl, mix egg yolks and sugar with a hand whisk. Pour hot milk mixture into the egg yolks/sugar preparation, constantly whisking. Bring the mixture back to the saucepan and cook over low heat, whisking.

Once the preparation thickens, remove the saucepan from heat and cool it down, placing it over a large bowl with the iced cold water. Add the drained gelatin immediately and mix until melted. Pour the warm preparation over the melted dark chocolate and gently mix with the spatula. Once the chocolate ganache reaches 113 F/45 C, add the cold whipped cream and mix with the spatula. Keep the ready chocolate mousse at room temperature.

To make chocolate decorations, temper dark chocolate. Cut the guitar sheets or simply plastic sheets into small pieces and arrange them on a table. Using the bent spatula, spread the small amount of chocolate on those sheets and form thin tears. Sprinkle chocolate with cocoa powder and place the sheets with chocolate in a baguette pan to give them a slight curvature. Instead, use small narrow glasses to insert sheets with chocolate.

To assemble the cake, flip the biscuit over parchment or a kitchen towel and remove the parchment paper or baking sheet. Cut the rectangle of 11 x 10 inch/28 x 25 cm. Spread a thin layer of the raspberry preserves and another chocolate mousse layer using the bent spatula. Roll the biscuit, starting from the top to the bottom, and tighten it. Wrap the biscuit roll in parchment paper and refrigerate for at least 1 hour. 

Take the roll cake out of the fridge and place it on a serving platter. Cover the roll with the chocolate mousse and spoon it with the back of a tablespoon. Decorate the cake with chocolate tears. Sprinkle with cocoa powder and icing (powdered) sugar. Arrange fresh raspberries. Refrigerate for at least 2 hours.