In a bowl, whisk together the sugar, cornstarch, and turmeric powder
Pour the milk into a pot and add the dry mixture and the lemon zest. Whisk to dissolve the sugar. Put heat on medium and keep mixing until it thickens.
Add lemon juice. Cook for 1 minute more while mixing.
Turn off heat, stir in vanilla, butter, and salt. Taste to adjust seasonings.
Pour through a fine-meshed sieve and let cool completely before using.