Preheat the oven to 170°C (325°F/Gas 3) and grease and flour a 23 cm (9 in) cake tin. Whip the egg whites to soft peaks in a spotlessly clean bowl with a pinch of salt. Slowly pour in 100 g (3½ oz/scant ½ cup) of the sugar while whisking. Mix the yolks with the remaining sugar in another bowl, until pale and creamy, then use a spatula to fold the two together. Sift in the two flours, add the lemon zest and fold them in. Pour into the prepared tin and cook for 30–35 minutes, until a skewer comes out clean when inserted into the centre. When cooked, remove from the tin and allow to cool on a rack. Meanwhile, make the filling by draining the ricotta and mixing the ingredients, except for the chocolate, together in a bowl.
Carefully cut a 1 cm (½ in) thick disc from the top of the cake and set aside. Next cut a 2 cm (¾ in) disc, leaving you with a final one of 1 cm (½ in). Lay one of the shallower discs back in the tin with the outside surface downwards. Next cut a circle from the thicker disc so that you are left with a ring of around 1 cm (½ in) – this will make the border around the hole for the filling. Place it into the tin and eat the centre as a cook’s treat! Mix together the ingredients for the syrup and brush two-thirds over the sponge.
Fill the hole with the ricotta filling mixture. Melt the chocolate briefly in a microwave or in a bowl over a saucepan of boiling water. Spread the melted chocolate over the final disc of sponge with a palette knife. Invert this over the cake and press down lightly. Brush on the remaining syrup. Leave the cake to rest in the fridge for at least 6 hours and up to a day. Turn out of the mould.
Whip the cream to form soft peaks and fold into the ricotta and honey or sugar. Use a palette knife to spread this evenly over the cake. Finish the coating with a sprinkling of nuts, grated chocolate and icing sugar. Chill in the fridge for a minimum of 1 hour or up to a day before serving.