Preheat your broiler or oven to its highest setting.
Arrange, in a single layer, the tomatillos, onions, garlic, and poblanos, skin side up on a sheet tray lined with parchment paper. You may need two sheet trays depending on the size of the vegetables.
Put the sheet tray on the highest tier in your oven so the contents are as close to the broiler as possible. Broil on high about 10-15 minutes until the vegetables have charred and caramelized.
Transfer the charred vegetables to a blender and blend on high for about 1 minute until smooth. Add in 25g (½ cup) of cilantro to the blender and continue to blend until fully broken down.
By this time, the chicken thighs should be finished cooking. Remove them from their cooking liquid and set them aside to cool slightly.
Pour the pureed tomatillo sauce into the pot with the remaining cooking liquid along with 25 grams (1 ¾ tablespoons) Better Than Bouillon chicken base.
Bring the mixture to a simmer and cook on medium-low for about 20 minutes, reduce the sauce by roughly 30-40% until it has thickened slightly and has developed a sweeter flavor.
Remove the pan from the heat and set aside.