Preheat the oven to 180°C / 350°F and line the baking tray with parchment paper.

Whip the 6 egg whites in a mixing bowl until stiff peaks form.

Beat the 6 egg yolks with the granulated sugar until pale and thick.

Mix the 3 tbsp flour, 3 tbsp ground walnuts, and 2 tbsp cocoa powder into the yolk mixture.

Fold the whipped egg whites gently into the batter until no white streaks remain.

Spread the batter evenly on the lined baking tray and bake for 15 minutes.

Whisk the wheat flour with a small amount of the 500 ml milk until smooth.

Heat the remaining milk in a saucepan and whisk in the flour slurry.

Cook the cream while stirring until thickened, then remove it from the heat.

Add the dark baking chocolate to the hot cream and stir until fully melted.

Cover the cream directly with plastic wrap and let it cool completely.

Turn the baked sponge onto a clean damp kitchen towel and keep the parchment attached.

Roll each long side of the warm sponge inward with the towel, then let it cool.

Beat the room-temperature butter with the powdered sugar until light and fluffy.

Beat the cooled chocolate cream into the butter mixture until smooth.

Unroll the cooled sponge and remove the parchment paper.

Spread most of the chocolate cream evenly over the sponge, reserving some for coating.

Place 2 peeled bananas along each long side of the sponge.

Roll both sides inward toward the center and place the roll seam-side down.

Coat the outside with the reserved cream and sprinkle with chopped walnuts.

Chill the Swiss roll for at least 120 minutes before slicing and serving.