For the Spicy Chipotle Mayo: Add all ingredients into a food processor and puree until smooth. Add salt and pepper to taste, if needed.
For the Panini: Turn the oven to broil. Place the tomato slices on a lined baking sheet. Drizzle with a little olive oil and sprinkle with salt. Place into the oven and roast the tomatoes until the top is slightly charred. Remove from the oven and set aside. (Note: This step may be skipped and replaced with slices of avocado.).
Heat a panini grill or grill pan on medium heat. Spread a little bit of butter on one side of each piece of the bread.
Evenly distribute the chicken, cheese, bacon and tomatoes (or avocado slices) on top of two slices, butter side down.
On the other pieces of bread, spread some of the chipotle mayonnaise on the side that is not buttered. Place on top of the sandwich. Place the sandwiches on the grill. If you are using a grill pan, place a heavy pan on top of each sandwich.
Cook for about 2-3 minutes until the bread is golden brown. Gently flip the sandwich over and cook for another 2-3 minutes until the other side is golden brown.