Preheat the oven to 200°C (400°F/Gas 6). Grease a 22cm (8½inch) spring-form cake tin.
Spread the walnuts on a baking tray and toast for 5 minutes. Use a sharp knife to roughly chop two-thirds of the walnuts. Set aside. Finely chop the remaining walnuts in a food processor and use to coat the inside of the cake tin in a thick layer. Reduce the oven to 190°C (375°F/Gas 5).
Cream the butter and 90g (3¼oz.heaped⅓cup) of the sugar in a large bowl with electric beaters until pale and fluffy. Add the egg yolks, orange zest, lemon juice, ricotta, flour and reserved walnuts. Gently mix until well combined.
Whisk the egg whites in a large bowl until soft peaks form. Gradually add the remaining sugar and whisk until stiff. Using a metal spoon, fold a large scoop of egg white into the ricotta mixture. Carefully fold in the remaining egg white. Spoon into the cake tin and level the surface. Bake for 35–40 minutes, or until the cake is set and the surface is golden. Cool in the tin for 15 minutes before turning out.
To make the candied zest, peel the orange, discarding the pith. Cut the zest into long, very thin strips. Put in a bowl, cover with boiling water and soak for 3–4 minutes, then drain and dry on paper towels. Combine the orange juice and sugar in a small saucepan, stirring to dissolve the sugar, then bring to the boil, add the zest and simmer for 5 minutes. Remove the zest with tongs and spread on a plate to cool.
To serve, dust the centre of the cake with the cocoa, leaving 5cm (2inches) around the rim uncovered. Arrange the zest around the rim.