Preheat oven to 180°C/360°F and place a rack in the middle. Grease and line a 9-inch springform cake pan with parchment paper.
Break 150g dark chocolate into small pieces and place in a microwave-safe bowl; microwave at 50% power in 15–30 second intervals, stirring after each, until smooth. Set aside to cool slightly.
In a large bowl, whisk 300g ricotta, 3 eggs, and 180g sugar until creamy and lump-free, about 4 minutes. Add the melted dark chocolate, 1 tablespoon fresh lemon zest (or orange zest) and a pinch of sea salt, and mix to combine.
Place a sieve over the bowl and sift in 185g all-purpose flour, 30g cocoa powder, and 1 tablespoon baking powder; whisk until just combined, taking care not to overmix.
Pour the thick batter into the prepared pan (it will be similar to brownie batter) and smooth the top with a spatula. If desired, sprinkle extra chocolate chips or chopped dark chocolate on top.
Bake for approximately 40 minutes, covering the top with foil during the last 10 minutes to prevent excessive browning. Insert a toothpick into the center — if it comes out clean, the cake is done.
Remove the cake from the oven and allow it to cool completely, then gently remove it from the pan.
Dust the cooled cake with powdered sugar and serve at room temperature; optionally top slices with vanilla ice cream, fresh cream, or Greek yogurt.