Preheat oven to 180°C/360°F and adjust a baking rack onto the middle shelf. Grease and line with parchment paper a 9-inch springform cake pan and set it aside.
Break the chocolate into small pieces, then place it in a microwave-safe bowl. Microwave on medium power (50%) for 15-30 second intervals.Stir after each interval until smooth and fully melted, set aside and let it cool slightly.
In a large bowl whisk the ricotta cheese, eggs and sugar until you reach a creamy and lump-free mixture, about 4 minutes. Then add the melted dark chocolate, lemon zest and a pinch of sea salt.
Place a sieve/ strainer over the bowl. Add the flour. cocoa powder and baking powder, and sift into the cake batter. Whisk until just combined (do not overmix).
Pour the cake batter into the prepared baking pan. The batter will be thick, similar to brownie batter, so use a spatula to smooth the top evenly. If desired, sprinkle extra chocolate chips or chopped dark chocolate over the top for added richness.
Bake in the oven for approximately 40 minutes. During the last 10 minutes of baking time, cover the top of the cake with foil to prevent it from browning too much. To check if the cake is ready, insert a toothpick in the center, if it comes out clean, the cake is ready.
Remove the ricotta chocolate cake from the oven, allow to cool completely, then gently remove the cake from the pan and serve.
Dust the cake with powdered sugar and serve at room temperature. It's lovely topped with vanilla ice cream, fresh cream, or Greek yogurt.