In a food processor or blender, grind 3/4 cup of the nuts to a paste. (Note: I couldn't get my pistachios to form a paste without adding a teaspoon or two of water).
In a saucepan, combine the sugar, cornstarch, and salt.
Add the milk and pistachios and bring to a boil over medium heat, stirring constantly for 5 to 7 minutes. Remove from the heat as soon as the mixture thickens. (Note: This took approximately 12 minutes for me on a gas stove).
Stir in the butter until melted. Add the vanilla. Let cool in refrigerator for 10 minutes.
Add 3 drops green food coloring and mix with a spoon to combine.
Pour into the pop molds. Insert the sticks. Freeze for at least 8 hours.
Remove from the freezer. Let stand at room temperature for 5 minutes before removing the pops from the molds. (Note: Be gentle! I found that running the molds under warm water also helped to neatly loosen them).