Place a dry skillet over medium heat. Remove stems from the guajillo and ancho chilies and shake out most of the seeds. Once the skillet is hot, add the cleaned chilies in a single layer. Press them gently against the surface with a spatula for 30-45 seconds per side until aromatic and fragrant. Don't let them smoke or char. Transfer immediately to a heat-safe bowl.

Pour hot water (just off the boil, around 180-200°F) over the toasted chilies to cover completely. Place a small plate on top to keep them submerged. Let soak for 15-20 minutes until soft and pliable. Drain in a colander, reserving about ¼ cup of soaking liquid. Gently squeeze excess water from the chilies.

Add the rehydrated chilies to a blender along with garlic, olive oil, orange juice and zest, apple cider vinegar, smoked paprika, cumin, Mexican oregano, salt, black pepper, and brown sugar if using. Add 2-3 tablespoons warm water or reserved soaking liquid. Blend on high for 1-2 minutes until completely smooth and glossy. The paste should be thick, similar to barbecue sauce consistency. Taste and adjust seasoning as needed.

Transfer the blended paste to a small saucepan with 1 teaspoon oil. Cook over medium-low heat, stirring constantly, for 3-4 minutes until the paste darkens slightly, thickens, and becomes more aromatic. The raw garlic will cook and the spices will bloom. Remove from heat and let cool to room temperature. This step is optional but recommended for best flavor.

Pat the peeled and deveined shrimp completely dry with paper towels. Reserve 2-3 tablespoons of the chili paste in a separate bowl for basting. Toss the shrimp with 3-4 tablespoons of paste in a medium bowl until evenly coated. Cover and refrigerate for 20-30 minutes. Don't marinate longer than 45 minutes or the acid will make the shrimp mushy.

Preheat your grill or griddle to medium-high heat (400-450°F). Lightly oil the cooking surface. Remove shrimp from marinade and place in a single layer on the hot surface. Cook 2-3 minutes per side without moving, brushing with reserved clean paste during cooking. The shrimp should develop charred spots and the paste should caramelize into a glossy glaze. Cook until opaque throughout and curled into a C-shape, reaching 120-145°F internal temperature.

Transfer grilled shrimp to a serving platter. Brush with any remaining reserved paste for extra gloss. Garnish with fresh cilantro and serve with lime wedges. Serve in warm corn tortillas with avocado crema, over rice with beans, or simply with lime wedges. The paste-glazed shrimp should be juicy, tender, and deeply flavored.