Lightly grease a 23 cm (9 inches) spring-form pan and line with parchment paper. (I like to line the bottom of the pan and grease the ring and sprinkle with a little flour-see post for more detail). Set aside. Preheat the oven to 350 F/ 180 C/ gas mark 4.
Melt the chocolate by placing it in a heat proof bowl over a pot of gently simmering water (the water must not touch the bowl). Stir as the chocolate is melting. Remove from the heat when the chocolate has melted almost completely (it will continue melting away from the heat). Set aside.
In a large bowl beat together the ricotta, cream cheese and half the sugar until fluffy (for about a minute). Gradually add the flour, cocoa and egg yolks until combined. Pour in the melted chocolate (once cooled) and continue beating until just combined.
In a separate bowl beat the egg whites until soft peaks start forming. Gradually add the remaining sugar and continue beating until glossy, fluffy and thick.
Gradually add the egg whites mixture (in 4 goes) into the cheesecake batter and fold in using a spatula until just combined. Pour the cheesecake batter into the pan and bake in the centre of the oven for 1 hour. After 40 minutes cover loosely with baking paper to prevent the cheesecake browning too quickly.
Remove from the oven and place on a cooling rack for 15 minutes. Run a spatula around the cake, remove the ring and leave to cool completely. (See post for tips on how to safely remove the cheesecake from the pan). Decorate with chocolate shavings (I used a vegetable peeler to make the shavings). Refrigerate for at least 30 minutes if possible before serving. Keep refrigerated.
When melting the chocolate remove it from the saucepan when it's almost melted. It will melt completely as you stir it.
Wait for the melted chocolate to cool (this will only take a few minutes) before adding into the cheesecake batter.
Scatter the chocolate shavings over the cake once it's cooled completely.
Refrigerate, covered, for up to 3 days.
See the post for tips on preparing the spring-form pan and safely removing the cheesecake from the pan.