1.Preheat oven to 180°C/160°C fan-forced. Line base and side of a 20cm (base diameter) springform cake pan with baking paper. Cut cake lengthwise into 1cm-thick slices; line base of prepared pan, trimming to fit neatly. Reserve leftover cake for another use.
2.Process ricotta and sugar until combined. Add eggs, orange zest, orange juice, lemon zest and essence; process until combined. Transfer to a large bowl; stir in chocolate.
3.Spoon mixture into prepared pan; smooth surface. Bake for 50 minutes or until almost set at centre. Cool in oven, with door ajar, for 1 hour (this prevents cracking). Chill overnight.
4.Dust cheesecake with drinking chocolate and serve with orange slices.
Leftover chocolate cake makes a great lunch box treat.