Place rind in a saucepan, cover with water and bring to the boil. Drain, then repeat process twice. Return rind to pan, add 110g (1/2 cup) sugar and 125ml (1/2 cup) water. Bring to the boil over medium heat, then simmer for 15 minutes or until syrup is thick and rind translucent. Drain and reserve syrup. Cool rind, then toss in extra sugar and chop coarsely.
Place dark chocolate in a heatproof bowl. Pour 150ml boiling water over and whisk until well combined.
Preheat oven to 180°C. Crush flaked almonds. Using an electric mixer, beat 150g butter, remaining 150g sugar and 1/2 teaspoon salt until pale and fluffy. Add eggs one at a time, beating after each, then beat in chocolate mixture. Sift over ground almonds and baking powder, add flaked almonds and orange rind, and stir to combine. Spoon into a 20cm greased and baking paper-baselined cake tin. Bake on bottom shelf of oven for 50-55 minutes or until a cake tester comes out clean. Stand cake in tin for 10 minutes, then turn out onto a wire rack to cool completely.
Stir orange chocolate, remaining butter and cream over a pan of simmering water until melted. Cool to the consistency of thick cream, then spread over cake. Leave to set.