Preheat the oven to 190°C (170°C fan assisted oven, 375 F, gas mark 5). First make the sponge. Grease and line an 18cm (7inch) square cake tin. Whisk the egg and sugar together until pale and thick, like softly whipped cream – this will take about 3 minutes.
Sift the flour on top and then fold in gently using a metal spoon, taking care not to knock the air out of the mixture.
Spoon the into the prepared cake tin, smooth the top and bake in the oven for 10-12 minutes, until risen, springy to the touch and lightly golden. Leave to cool for 5 minutes then carefully turn out on to a wire rack, peel away the lining paper and leave to cool completely, baked side uppermost.
Line a deep 500g (1lb 2oz) loaf tin with cling film, smoothing out the cling film as much as possible round the tin sides of the tin. Trim the sponge so that it fits snugly into the bottom of the tin (if you cut the sponge carefully you should have enough sponge to make another one. Otherwise, keep the trimmings for truffles or cake pops). Set aside.
Now make the layers. Break up 100g (3 ½ oz) White Chocolate into a heatproof bowl and place over a saucepan of barely simmering water to melt. Remove the bowl from the water.
Quickly stir in 75g (3oz) ricotta and the Vanilla Extract into the chocolate – the mixture will thicken quickly – and mix until well blended. Pile on top of the sponge base and spread out evenly, tapping the tin on the work surface to ensure the chocolate cream settles evenly. Chill for about 30 minutes until firm.
Melt the remaining White Chocolate as above. Put the ricotta cheese in a bowl and mix in the Limoncello and as much Yellow Food Colour Gel as required to make a lemony colour – for a very rich colour, use the whole tube. Beat until well blended together then add the melted chocolate and mix well.
Pile over the white chocolate layer and smooth over evenly, tapping the tin on the work surface to make sure there are no gaps. Chill for at least 2 hours until completely set.
To serve, invert the tin on to a serving plate or board and peel off the cling film. For a very smooth finish, gently run the warm blade of a knife (hold in hot water for a few seconds and dry) over the sides. Dust lightly with icing sugar and decorate with lemon rind, then slice and enjoy!
To make the lemon rind twists, using a vegetable peeler, slice off the rind, working round the fruit in order to make a long strip. Trim the lemon rind to the desired width and length, then gently twist it into spirals by wrapping it round a chopstick or clean pencil. Carefully slide the rind off the stick or pencil and the twist should stay in place.