Make an oleo-saccharum: peel two lemons and two oranges. In a medium bowl, muddle the brown sugar with the peels until the sugar is damp, cover with plastic, rest overnight. In a large enough to hold all the liquid sized container, add the milk then juice the lemons and oranges into the milk. Add the Port and Cognac. Add the tea to the oleo saccharum to dissolve the sugar, then strain that into the rest of the liquids. Stir to combine and to make sure it curdles and let rest in the refrigerator overnight. The next day, strain, then fine strain through a coffee filter into a clean container. This should be crystal clear. Refrigerate. To serve, add the punch base and the sparkling wine to a large punch bowl. Grate nutmeg, float some sort of decorative chunk of ice in the middle.
This should make 18 cups of punch, so 144 oz, which I'd say is 30 servings at about 22 proof.
This is a tear down and rebuild of the Black-Tea Port Milk Punch from Splendid Table, which itself was based on Jerry Thomas' Ruby Punch.