For the cake, grease the cake tin with the two teaspoons of butter. Sprinkle the tablespoon of almonds into the tin and shake until the inside of the tin is completely coated with a light dusting of almonds.
Melt the chocolate in a heatproof bowl set over a pan of simmering water. Remove the bowl from the heat and stir the chocolate until smooth, then leave to cool for at least 15 minutes.
Preheat the oven to 190°C/375F/Gas 5.
Put the butter and icing sugar in a large bowl and cream them together until smooth and light. Add the egg yolks, one at a time, beating well between each addition. Beat in the cooled chocolate, then the 50g/2oz of ground almonds, flour and baking powder.
Whisk the egg whites in a large, clean bowl with an electric hand-whisk until they are stiff but not dry. Stir about two heaped tablespoons of the egg whites into the chocolate mixture to loosen it, then gently fold in the rest with a metal spoon.
Pour the mixture into the cake tin and bake in the centre of the oven for 35–40 minutes, until it is well risen and beginning to shrink away from the sides of the tin. Leave the cake to stand for 2 or 3 minutes, then gently run a knife around the sides and turn it out on to a wire rack to cool.
To make the icing, melt the chocolate with the butter and syrup in a heatproof bowl over a pan of simmering water. Remove from the heat and stir until smooth, then leave to cool for 5 minutes.
Slowly drizzle the icing over the cake and allow it to slide slowly down the sides. If the icing moves too quickly or doesn’t cover the cake properly, let it cool for a few more minutes and then try again. Work slowly to get the best coverage. Leave the iced cake to stand for 10 minutes, then scatter with the almonds and sugar crystals if using.
Set aside for an hour or so until the icing is set before serving.