Heat the oven to 180 C / 350 F. Butter a 26 cm / 10-inch springform pan and dust it lightly with flour or fine semolina, tapping out the excess.

Pour the milk and water into a 3-quart / 2.8 L saucepan. Add the butter and salt, and bring to a gentle boil over medium heat.

Reduce the heat to low. Sprinkle in the semolina in a thin, steady stream, whisking constantly so it doesn't clump. Cook for 5 to 8 minutes, stirring, until the porridge is thick and pulls away from the sides of the pan.

Scrape the porridge into a large bowl and let it cool for 15 to 20 minutes, stirring once or twice, until just warm.

In a separate bowl, whisk the ricotta smooth. Add the sugar and whisk until creamy, about 1 minute.

Whisk in the eggs one at a time, then stir in the lemon zest, orange zest, vanilla, and cinnamon if using.

Fold the ricotta mixture into the cooled semolina porridge in two additions, mixing until the batter is smooth with no streaks.

Pour the batter into the prepared pan and smooth the top with a spatula.

Bake for 55 to 60 minutes, until the top is deep gold and the edges pull slightly from the pan. The center should still jiggle slightly, like a set custard, when you nudge the pan.

Cool the cake in the pan on a wire rack to room temperature, about 1 hour, then refrigerate for at least 2 hours before slicing.

Dust with powdered sugar just before serving. Cut into 12 wedges with a thin knife.