Remove the woody outer leaves of the lemon grass stalk. Finely slice, crosswise, the tender inner stalk and transfer to a blender. Add the peanuts, garlic, ginger, lime rind, lime juice, anchovy paste, sugar and red pepper flakes.
Add coconut milk and puree into a fine puree. Transfer this mixture to a food processor and puree with basil and cilantro. Season to taste with salt. Pack in half or cup portions and freeze.