In a large pot, heat the milk together with sugar and vanilla. Gradually add the semolina and stir continuously at a low temperature until the cream thickens. Remove the pot from the stove and add the egg yolks to the cream. Add the butter and the orange zest and stir until all the ingredients are mixed well. Add the beaten egg whites to the meringue with very little salt. Butter a baking pan and spread the mixture evenly.