Fill a large pot 3/4-full with water.

Place the unopened can of sweetened condensed milk in the water, ensuring it is completely submerged.

Bring the water to a gentle boil.

Reduce heat and simmer for 3 hours, adding more water as needed to keep the can covered.

Remove the pot from the heat and allow it to cool until safe to handle.

Monitor the can for bulging; if bulging occurs, remove from heat to cool.

Once cooled, carefully puncture a small hole in the top of the can.

Return the can to the pot, partially submerged, and continue cooking if needed to achieve desired color.

After a total of 3 hours of cooking, remove and let cool completely before opening.

Alternatively, cook the condensed milk out of the can using a double boiler, stirring every 5 minutes for 3-4 hours until golden brown.

Or purchase canned dulce de leche from the store.

Once dulce de leche has cooled, prepare the ice cream base.

Combine heavy cream and milk in a medium heavy saucepan and bring to a gentle boil over medium heat.

In a heatproof large bowl, whisk egg yolks, 3/4 of the dulce de leche, and granulated sugar until smooth and pale.

When the milk boils, whisk 1 cup of the hot cream mixture into the egg yolk-sugar mixture until smooth.

Gradually add the egg mixture in a slow, steady stream to the hot cream in the pan, whisking constantly.

Cook over medium-low heat, stirring constantly, until the mixture thickens enough to coat the back of a spoon (about 5 minutes).

Remove from the heat and strain through a fine mesh strainer into a clean container.

Cover with plastic wrap, pressing down against the surface to prevent a skin from forming.

Refrigerate until well chilled, about 2 hours.

Remove from the refrigerator and pour into the bowl of an ice cream machine.

Freeze according to the manufacturer's instructions.

When the ice cream is nearly completely churned, spoon in the remaining dulce de leche to create a swirl.

Transfer to an airtight container and freeze until ready to serve.