NOTE: If you can't find vanilla beans, use a high quality vanilla extract.
1 quart milk
2 cups heavy cream
2 vanilla beans, split in half
1/2 cup granulated sugar
6 large egg yolks, lightly beaten
14 ounces almond paste (marzipan),
cut into 1/4-inch cubes
In a large heavy saucepan: combine milk, heavy cream, vanilla beans and sugar. Simmer over low heat until the mixture begins to steam (half an hour).
Place beaten egg yolks in a medium bowl. Whisk in a bit of the hot milk mixture. Stir egg/milk mixture back into large saucepan. Continue cooking over low heat, stirring constantly until mixture forms a thin custard (5 to 15 minutes). Remove pan from heat. Scrape the black vanilla seeds inside vanilla beans and mix into custard. Throw away empty vanilla pods. Let the custard cool for 15 minutes.
Fit a food processor with a metal blade. Put diced marzipan into food processor. Add one sixth of the warm custard mixture and pulse until the marzipan softens (4 or 5 times). Add another one sixth and blend until smooth (one minute). Pour marzipan custard mixture into plain custard mixture. Mix well. Pour the mixture into a a large pan, or two 9 x 5 x 3-inch pans and freeze the mixture until it is mushy. This will take at least 4 hours, because of the richness of the ice cream.
Empty the ice cream mixture into a large electric mixer bowl, and beat 2 minutes at the highest speed, until fluffy. Pack into a 1/2 gallon plastic freezer container and freeze until semi-firm.