1 bottle (750 mL) brandy, preferably cognac or Armagnac
1 bottle (750 mL) aged or dark rum
3 lemons, cut into thin rounds
1 cup whole milk
1 cup demerara sugar
7 cups water
Combine the brandy, rum and lemon slices in a large container with a tightfitting lid. Let steep at room temperature for 24 hours.
Heat the milk in a small saucepan over medium to medium-high heat so that it comes to a gentle boil (between 200 and 212 degrees), stirring constantly.
Add the milk to the brandy mixture, along with the sugar and water, stirring to dissolve the sugar and mix well.
Strain the mixture twice into a large pitcher or bowl: first through a fine-mesh strainer, then through several layers of cheesecloth, a clean dish towel or a coffee filter, until all the solids have been separated. Discard the solids.
Serve in a punch bowl over ice, or serve warm from a large pot kept on low heat.
Serves 18 to 24.
Adapted from "The Punch Bowl" by Dan Searing (Sterling Epicure, 2011)