Add the water and sugar in a small pot and bring to a boil. Reduce heat to low and sift in the cocoa powder.
¾ cup water, 1½ cups granulated sugar, ¾ cup Dutch-process cocoa powder
Then whisk in the vanilla, salt, and corn syrup and continue to cook for about 10 minutes, whisking often, until it starts to thicken. Remove from heat and strain through a fine mesh sieve to remove any clumps. Allow it to cool before using.
1 tablespoon vanilla extract, ⅛ teaspoon sea salt, 1 tablespoon light corn syrup
Stir together ¼ cup of cooled chocolate syrup and milk until fully combined.
¼ cup chocolate syrup, 1 cup whole milk