Trim any fat from chicken and salt and pepper both sides of each chicken breast, set aside.
In a shallow bowl combine olive oil, balsamic vinegar, brown sugar, chili powder, unsweetened cocoa powder, smoked paprika, cinnamon, and garlic powder and stir until a paste forms. Heavily coat both sides of each chicken breast.
Heat a grill to 400 degrees F. Once the grill gets hot, place the prepared chicken breasts over flame and cook for about 5-7 minutes each side or until internal temperature reaches 165 degrees. For added flavor brush the marinade over the chicken while cooking. The chicken will start to turn an amber color with some charring. If the chicken is getting too charred for your liking move the chicken to a cooler part of the grill until cooked all the way through.
Once cooked all the way through, remove the chicken from the grill and dice into small chunks.
Wrap the tortillas in a few damp paper towels and warm in microwave for 20 seconds. Take warm tortilla and spread with a small amount of plain Greek yogurt. Then add lettuce, diced chicken and any other additional toppings. Drizzle with a small amount of Cilantro Lime Butter and serve warm. Enjoy!
Cilantro Lime Butter
Combine all ingredients in small bowl and stir to combine. I like to do this right before serving so the butter doesn't cool and separate. Drizzle over completed Grilled Chicken Tacos.
Alternatively you can bake the chicken in the oven at 375 degrees for 30 minutes or until internal temperature reads 165 degrees. The flavor develops better, in my opinion, on the grill; but if the weather isn't great or you do not have access to a grill the oven works perfectly fine as well.
Store leftover grilled chicken tacos in an airtight container for up to 4 days and reheat when ready to eat.