In a saucepan, combine the heavy cream, butter, ¾ cup allulose, vanilla, and salt over medium-low heat.
Cook, stirring frequently, 15–20 minutes, until thickened and lightly caramel-colored.
Divide the manjarblanco among 6 small glasses or ramekins and chill at least 1 hour.
Whip the egg whites to soft peaks, then gradually add the ¼ cup allulose, beating to stiff, glossy peaks.
Fold the reduced port wine gently into the meringue.
Top each chilled manjarblanco with a generous spoonful of the port meringue.
Dust with cinnamon and serve immediately.