I used to make the Kuri An: https://cookpad.com/en/recipes/170074-kuri-an-sweet-chestnut-paste
Prepare three bowls: one small, one medium, and one large. Put the vegetable based whipped cream in the large bowl, the egg white in the medium bowl, and the egg yolk in the small bowl.
Add the Kuri An (Sweet Chestnut Paste) and rum to the egg yolk in the small bowl, and mix well with a whisk.
Beat the egg white in the medium bowl at high speed with a hand mixer. Once it thickens, beat in 10 g sugar in two batches to make a stiff meringue.
Add the remaining sugar and skim milk powder to the whipped cream in the large bowl, without washing the beaters of the hand mixer. Whip until soft peaks form.
Add about half of the meringue to the egg yolk and sweet chestnut paste mixture from Step 3, and mix it well with a whisk.
Fold the remaining meringue from Step 4 and the mixture from Step 6 into the whipped cream mixture with a spatula. Add vanilla essence to taste, and chopped sweet chestnuts.
Put in the freezer to set. (No need to stir it before it sets.)
Here is a marbled version using "Chocolate Ice Cream". This is also really good. https://cookpad.com/en/recipes/148806-chocolate-ice-cream
Here, I mixed in 10-15 g starch-based sweet syrup (sugar syrup) to the mixture in Step 7 before freezing. I think this is very close to the taste of "Sato no Kuri" ice bars...
The amount of syrup you use depends on the firmness of the an paste. The an paste should be creamy.