Begin by finely chopping the garlic, shallots, and tender light-yellow parts of the lemongrass. In a mixing bowl, combine these ingredients with fish sauce, dark soy sauce, sugar, and water to create a cohesive marinade.

Place the boneless chicken thighs into the bowl with the marinade, ensuring each piece is fully coated. Cover the bowl with plastic wrap and let it rest at room temperature for 30 minutes to 1 hour.

Once the chicken has marinated, carefully remove it from the bowl, allowing excess marinade to drip off. Gently scrape off most of the aromatics.

Preheat your grill to medium heat, around 350°F (175°C). Place the chicken thighs skin-side down on the grill and sear for 2-3 minutes.

Flip the chicken and cook for an additional 5-8 minutes, ensuring they're cooked through to an internal temperature of 165°F (74°C).

Remove the chicken from the grill and let it rest for about 5 minutes. After resting, slice the chicken into bite-sized pieces for serving.

In a small bowl, combine minced garlic, fresh lime juice, hot water, sugar, and fish sauce to prepare the nuoc cham dipping sauce. Stir until the sugar has dissolved.

Serve the chicken slices warm with the homemade nuoc cham on the side, paired with steamed rice or vermicelli noodles.