Toast the bread in a toaster, or under a broiler on both sides.
Arrange 3 bread slices in a row. Spread 1 tablespoon mayonnaise over 1 side of each bread slice. Place a lettuce stack on top of the first bread slice, top with 2 tomato slices, and season with salt and pepper, to taste. Place 2 slices bacon over the tomatoes (broken to fit neatly if necessary) and top with the chicken (without letting any hang over the sides).
Add avocado slices & season with a pinch of salt and pepper, to taste. Repeat with the second bread slice. Carefully place the second layered bread slice on top of the first layered bread, chicken side-up. Cover with the third bread slice.
Pin the sandwich's layers together by piercing them with 4 frill picks or cocktail swords through the top bread slice, in 4 places in a diamond-like pattern, all the way to the bottom bread slice.
Using a serrated knife cut sandwich, diagonally, into 4 triangular pieces (each piece should be secured in the center with a pick or sword).