Preheat your oven to 180°C (350°F). Line a baking sheet (approximately 25x38 cm) with parchment paper, allowing the edges to hang over for easy removal later.
In a large bowl, whisk the eggs with the sugar for about 5 minutes until the mixture looks light and creamy.
Sift the flour, cocoa powder, baking powder, and salt into the egg mixture and gently fold the dry ingredients in with a spatula until just combined.
Stir in the melted dark chocolate and water until the batter is smooth and homogeneous.
Pour the batter into the prepared baking sheet and spread it evenly. Bake for 12-15 minutes until it springs back when lightly pressed.
Remove the cake from the oven and let it cool in the pan for about 5 minutes. Carefully lift the cake using the parchment paper and transfer it to a cooling rack to cool completely.
While the cake cools, whip the cream with a hand mixer until soft peaks form. Add the icing sugar and vanilla extract, and continue whipping until stiff peaks form.
Place the completely cooled cake on a large piece of parchment paper. Spread a layer of whipped cream evenly over the cake, leaving a small border at the edges.
Starting from one of the short ends, carefully roll the cake up using the parchment paper to form a log shape. Be sure to roll tightly but gently to avoid cracks.
Wrap the rolled log in the parchment paper and refrigerate for at least 30 minutes to set.
If needed, prepare additional whipped cream for frosting. Unwrap the log and decorate the outside with more whipped cream.
Use a fork to create textured bark effects on the frosting. Sprinkle with edible glitter for a festive touch if desired.
Slice the log and serve on a platter, garnished with fresh berries or mint leaves if desired.