Add cashews, milk, sugar, cornstarch, 3 teaspoons of vanilla extract, and the salt to a blender and blend until completely smooth.
Pour the mixture into a saucepan over medium high heat. Bring to a boil, whisking frequently.
Reduce heat to medium low and simmer until it thickens, about 15 minutes, whisking frequently so it doesn't stick or form clumps. Remove from heat and let cool at room temperature, then cover and refrigerate until ready to serve.