For the base:
175g crunchy cocoa biscuits, anything with a classic biscuit consistency will do.
Crush the biscuits (in a bag, with a rolling pin, or in a food processor) until fairly fine crumbs with some larger bits. Cut the butter up and melt it in a bain-marie or microwave. Add the butter to the biscuits (in a bowl!) and use a spoon to combine thoroughly.
Butter your cheesecake tin (20 cm springform tin) thoroughly and line with greaseproof paper (on the bottom will do). Put the butter-biscuit mixture in the bottom of the tin and press down until it forms an even and compact covering over the whole base of the tin, about 1cm deep. Use the back of a spoon or your knuckles to press it down.
Put it in the fridge for 30 mins while you prepare the filling.
For the filling:
120g dark chocolate, 50% cocoa
250g greek yoghurt
3 eggs (if very big, use 2)
100g granulated sugar
35g cocoa powder
optional: grated rind of one lemon
cocoa for dusting the top of the cake
Break the chocolate into pieces and melt in a bain-marie or microwave. Separate the egg yolks from the egg whites, and whisk the yolks with the sugar. Separately, whisk the whites with a pinch of salt until they form firm peaks.
Turn out the ricotta into a large bowl and cream it for a moment or two using a wooden spoon (add the grated rind of one lemon at this stage if you like!), then add the yoghurt gradually, followed by the egg yolks/sugar mixture, the melted chocolate and finally the whipped egg whites.
Pour the mixture into the tin, in which you have already prepared the biscuit base. Smooth the top, and bake in the centre of the oven at 160°C for about 50 minutes.
When done, remove and leave to cool in the tin. When it is lukewarm, place it in the fridge to chill for at least 2 hours.
When chilled, remove from the tin, dust with cocoa powder and serve, or if you like first decorate it with petals made of white chocolate and lemon slices, as below...
On a piece of greaseproof paper, sketch out petal shapes with a pencil.
Break up 75g white chocolate and melt in bain-marie or microwave. Using a spoon, fill in the petal shapes you've drawn on the paper with white chocolate. Leave to cool for a few minutes, then transfer to the fridge to harden up (at least 30 minutes).
Meanwhile cut the lemon into slim slices and simmer for 10 minutes in a pan with 100 ml water and 100g granulated sugar. Remove the lemon slices and cool them on a piece of greaseproof paper.
Place one of the lemon slices on top of the cake. Around it, arrange the white chocolate petals.If you like, outline them with a little dark chocolate sauce or melted dark chocolate.
Serve the remaining lemon slices together with the cake, if you like.
Keep the cake in the fridge :-)