Prepare the ice cream custard: In a bowl, beat egg yolks to blend.

In a 3- to 4-quart pan over medium-high heat, combine milk, cream, sugar, and coarsely chopped coffee beans; stir until sugar is dissolved and mixture is simmering.

Remove from heat, cover, and let stand for 30 minutes to infuse the coffee flavor.

Pour the mixture through a fine strainer into a bowl; discard the coffee beans.

Rinse the pan, return the milk mixture to it, and bring to a simmer over low heat.

Whisk 1/2 cup of the warm milk mixture into the beaten egg yolks; then pour the yolk mixture into the pan with the remaining milk mixture.

Stir constantly over low heat until the mixture is thick enough to coat the back of a spoon, about 4 to 6 minutes; be careful not to boil.

Pour the custard into a clean bowl and chill, stirring occasionally, until cold, about 2 hours; if desired, cover and chill up to 1 day.

Stir rum and vanilla into the chilled custard.

Freeze the mixture in a 1-quart or larger ice cream maker according to the manufacturer's directions.

While the ice cream is still soft, stir in the dark chocolate chunks.

Transfer the ice cream to an airtight container and freeze until firm, at least 6 hours, or up to 1 week.

Prepare the dulce de leche: In a heavy 5- to 6-quart pan over medium-high heat, stir 4 cups whole milk and 1 1/4 cups sugar until the sugar is dissolved and the mixture is boiling.

Stir in 1/2 teaspoon baking soda.

Reduce heat to low and simmer, stirring occasionally with a flexible spatula, until the mixture is golden brown and reduced to about 2 cups, about 1 1/2 to 2 1/2 hours.

Pour through a fine strainer into a bowl; discard any residue if present.

To serve: Scoop the ice cream into bowls and top with dulce de leche.