Combine 1 cup of the milk with the sugar in a medium saucepan.
Add in the zest and limoncello.
Heat mixture on medium until sugar is dissolved, stirring frequently.
Remove from heat and add in remaining milk.
Pour into a container and refrigerate overnight.Freeze in an ice cream maker according to manufacturer's instructions. It will probably stay soft because of the alcohol. You can place the ice cream in the freezer for another couple of hours for it to firm up more.For the amaretti:Preheat the oven to 300 degrees F. Line a couple baking sheets with parchment paper or silicone baking mats.
Combine almonds, sugar, flour, and salt in a food processor. Process until mixture is a fine meal. Turn out mixture into a large bowl.
Add the egg white and salt. Fold in with a spatula until mixture is fully combined. Drop dough by teaspoonfuls onto baking sheet about 1 1/2 inches apart.
Bake 18 to 20 minutes, rotating sheets about halfway through.
Remove from oven and let cool on wire racks for about 5 minutes before removing with a metal spatula. Finish cooling directly on wire racks.