Preheat the oven to 200C / 180C Fan. Grease and line a 30x40 cm (12x16-inch) rectangular baking tray with non-stick baking paper. Set aside.
In a large mixing bowl, sift together the ground almonds, icing sugar, flour and cocoa powder.
Add the whole eggs and, using an electric whisk, whisk for about 10 minutes - or until pale and airy.
In a separate bowl, whisk the egg whites with the caster sugar until soft peaks form.
Gently fold the whisked egg whites into the egg-flour mixture in batches - take care not to deflate the batter.
Carefully spread the batter onto the prepared baking tray - it should be about 1 cm thick. Bake in the oven for 7–10 minutes - or until the surface is just set and lightly golden. Leave to cool completely.
Meanwhile, prepare the cream: pour the cream into a large mixing bowl (ideally chilled) and add the icing sugar. Whisk until medium peaks form - take care not to over whisk. Cover and chill in the fridge until needed.
To assemble: remove the cooled cake from the tin and cut lengthwise into 3 equal-sized long strips.
Place one strip of cake on a flat surface lined with cling film. Using the back of a spoon or a palette knife, spread a thin layer of the whipped cream on top. Using the cling film to guide, roll the cake into a tight spiral.
Attach the second strip of cake by pressing the edge against the roll. Spread whipped cream on the second strip and continue rolling. Repeat with the third strip. Place onto a serving plate.
Next, make the ganache: put the chopped chocolate into a heat-proof bowl. Add the double cream and set over a pan of gently simmering water - make sure the bottom of the bowl doesn’t come into contact with the water. Stir gently until melted and smooth. Alternatively, pop the chopped chocolate and cream into a microwave-safe bowl and heat in 20-second bursts, stirring between each one, until melted and smooth. Set aside to cool and thicken slightly before decorating (this is normally between 5-15 minutes depending on the temperature of your kitchen.
To decorate: spread the ganache evenly over the sides of the rolled cake. Use a fork to create a wood/bark pattern for a rustic effect. I like to leave the top ganache-free so you can see the beautiful cream swirl. Decorate with fresh berries, chopped pistachios and sprigs of fresh mint. Enjoy!