Preheat the oven to 160 C. Butter an 11-inch round cake pan.
Heat the milk in a medium saucepan over medium heat until simmering, then whisk in the semolina and salt. Boil, stirring constantly, for 2 minutes.
Remove the saucepan from the heat, and stir in the sugar and butter until combined. Transfer the batter to a bowl to cool it down to warm, stirring frequently to prevent a skin from forming.
Zest the lemons, using only the yellow part, and stir into the batter. Beat in the eggs, one at a time, until combined. Beat in the ricotta.
Pour the batter into the pan, and bake until set and golden in places, about 1 hour. Transfer the pan to a rack to cool, and sprinkle with icing ’ sugar while still hot. Once the cake has cooled, sprinkle with icing sugar once more, then serve.