Roast fresh chilis in an oven preheated to 180°C (356°F) for about 10 minutes until they're slightly dehydrated and fragrant. Let them cool completely before handling.

Finely chop garlic, red shallots, and galangal (if using). Cut lemongrass into small segments.

Using a food processor, blitz the cooled lemongrass and roasted chilis separately.

In a saucepan, gently heat cooking oil with annatto seeds on low for 3-5 minutes until the oil takes on a vibrant red hue; strain out the seeds.

Warm some oil over low heat with star anise and Chinese cinnamon for about 2 minutes. Remove spices, add chopped garlic and shallots, and cook until the garlic is lightly golden (about 1 minute).

Mix in lemongrass, chilis, dried chili flakes (if using), fish sauce, salt, sugar, and chicken bouillon powder. Simmer over low heat for about 10 minutes.

Stir in the vibrant annatto-infused oil, mixing well for an additional minute, then set it aside to cool.

Transfer your homemade Sa Tế into airtight containers. It can be refrigerated for up to a month.