In a large skillet, cook the bacon until browned but not crisp, about 8 minutes; drain. In a bowl, stir together the mayonnaise, garlic and lemon juice; reserve 1/2 cup. Stir the chipotle chile, adobo sauce and cilantro into the remaining mayonnaise mixture; add the chicken and toss.
Place the toasts on a work surface and top 4 slices with some of the chicken salad and watercress. Spread some mayonnaise on both sides of another 4 slices of toast and set on top of the watercress; top with the avocado slices and bacon. Spread the remaining mayonnaise on the remaining 4 toast slices and set into place.
Slice each sandwich into 4 triangles and secure with toothpicks.