Preheat the oven to 200°C/ gas mark 6. Grease a 24cm × 30cm Swiss roll tin with a little sunflower oil, then line with non-stick baking paper.
Place the eggs and sugar in a food mixer and whisk for about 5 minutes, until thick and holding its shape. Add the vanilla extract and mix again to combine. Sift the flour, cocoa powder and baking powder on top and fold through very gently until well combined. Pour into the prepared tin and spread it evenly. Pop into the oven to bake for 10 to 12 minutes, until cooked through and just springy.
Meanwhile, prepare the icing. Place the chocolate in a medium-sized heatproof bowl. Melt over a pan of simmering water, not allowing the bowl to touch the water, or in 30 second blasts in the microwave, stirring until smooth. Remove and leave to cool.
Place the icing sugar in a food processor and blitz to remove any lumps. Add the butter and process until smooth. Add the cooled melted chocolate and the vanilla extract and blend again until smooth. Alternatively you can do this by hand, making sure to sift the icing sugar first.
Remove the sponge from the oven. Lay a large piece of nonstick baking paper on a surface and dust the teaspoon of icing sugar all over it. Carefully turn the sponge out onto the paper, peel away the original paper and discard. Roll the sponge up in the paper as you would a Swiss roll and leave to cool completely. Don’t worry about any cracks, as they will be covered over with the icing. To assemble, unroll the Swiss roll and spread 250g of the icing evenly over. Starting from one of the short sides, roll the sponge up to enclose the filling. Sit, seam side down, on a serving platter or board. Spread the remaining icing evenly all over, making a couple of little branch stumplike bumps in places. Using the tip of a small palette knife, kitchen knife or fork, mark all along the surface to create a bark-like effect.
To decorate, sprinkle the pistachios over in patches to resemble moss. Arrange the toasted flaked almonds in flower-like clusters around the log. Lightly dust with icing sugar and decorate with strings of redcurrants. This will keep in the fridge, covered, for a day or two.