Whisk together all of the ingredients for the marinade. Pour the marinade into a large zip lock bag and add the shrimp. Seal the bag and toss to coat the shrimp. Place in the fridge and marinate for at least 15 minutes or up to overnight.
After you get the shrimp in the fridge, prepare the poblanos by cooking them on the grill for about 5 minutes over high heat, turning throughout to ensure even cooking. When they are charred and blistered, remove them from the grill and run them under cold water for about 15 seconds. Pat dry with a paper towel and peel off the outer skin (this does't have to be perfect, just pull off most of the skin that comes off easily).
Chop the top of the pepper off and discard the stem and remove the seeds. Dice the poblanos. Add diced poblanos and all other sauce ingredients to a blender or food processor. Pulse until smooth. Cover and chill until ready to use.
When you have marinated the shrimp for as long as you can, dump out the marinade and skewer the shrimp and grill them for about 3-4 minutes on each side until they turn pink and begin to char very slightly. Drizzle with poblano sauce and serve immediately. Top with additional chopped cilantro if desired.