In a medium sauce pan, combine the mushrooms, 1 ½ cups of half and half, ½ cup brown sugar, vanilla, and salt.
Heat on medium until the mixture is almost boiling. Stir frequently.
Meanwhile, in a small mixing bowl, beat together the egg yolks and ¼ cup brown sugar.
When the mushroom and cream mixture is almost boiling, reduce the heat and stir in the egg yolk mixture.
Stir frequently. Heat until the temperature reaches 175°F. (A candy thermometer is a great tool to have.)
While the mixture is coming to temperature, add ½ cup half and half and the maple syrup in a medium mixing bowl.
When the mushroom mixture is at 175°F, remove from heat. Transfer the mixture through a strainer into the medium mixing bowl with the cold half and half and maple syrup.
Stir to mix well.
Chill for 4 – 24 hours.
Churn in an ice cream maker following the manufacturer’s instructions until it’s the desired consistency.