1Carefully remove stems and seeds from the Guajillo chile and Ancho chile. Lightly toast the chiles on a dry comal or pan for 30 seconds per side until fragrant. Place the toasted chiles in a bowl and cover with hot water, letting them rehydrate for 20 minutes until softened.
2While chiles rehydrate, peel and roughly chop 15g of the Garlic and all of the White onion. In a blender, combine the rehydrated chiles (reserving a small amount of the soaking water), chopped Garlic, White onion, 30g of the Lime juice, Cumin powder, Mexican oregano, and 3g of the Salt. Blend until very smooth, adding a splash of the reserved chile water if needed to achieve a pourable consistency. Strain the marinade through a fine-mesh sieve into a bowl, pressing on solids to extract all liquid.
3Peel and devein the Shrimp. In a bowl, toss the Shrimp with the prepared marinade, ensuring each shrimp is well coated. Let marinate for at least 15 minutes at room temperature.
