Remove about half the seeds from the red chilies and bird’s eye chilies to moderate the heat. Add the chilies and garlic to a food processor and pulse until coarsely chopped.

In a small saucepan over medium heat, combine the sugar, rice vinegar, water, fish sauce, and salt. Stir well and bring the mixture to a gentle simmer until the sugar has fully dissolved.

Stir in the chili-garlic mixture and continue to simmer for about 5 minutes, stirring occasionally, until the sauce slightly reduces and the flavors meld.

Mix the cornstarch with water to create a slurry, then add it to the saucepan. Stir immediately and cook for another 30 seconds until the sauce thickens to a syrup-like consistency.