In a large skillet, melt the coconut oil over medium heat until shimmering. Add the chicken and cook until thoroughly browned, about 6-8 minutes, stirring occasionally.
While the chicken is cooking, arrange the dried chiles on a baking sheet and place them under the broiler. Cook until the surface of the chilies blister, less than 5 minutes. Keep a close eye on them, they will burn quickly. Transfer the chilies to a spice mill or food processor and blitz until they become chili flakes. Pour the flakes into a small bowl and set aside.
Once the chicken is cooked, take the skillet off of heat and add the fish sauce, lime juice, sugar, toasted rice powder and 1 -2 teaspoons of the chili flakes, depending on your heat tolerance. Toss to coat the chicken and transfer to a large bowl.
Add the shallots, lime leaves, basil, culantro and mint to the bowl. Toss to integrate and set aside.
Fill a 9-inch pie plate with luke warm water. Place a rice paper sheet in the water and soak until slightly softened. Transfer the sheet to a wooden cutting board and 2-3 tbsp of the finished larb in the center of the sheet. Fold the sides over the larb and roll the sheet away from you as you would a burrito. Repeat with the remaining rice paper sheets and larb.
Transfer the finished rolls to a platter and serve with hoisin sauce and sriracha. Add cucumber chunks to the platter if desired. Serve immediately.