In a medium bowl, whisk together mayonnaise, cooked bacon, buttermilk, tarragon, chives, dill, garlic powder, onion powder, and parsley. Store in an airtight container for up to 3 days.
Spread a thin layer of mayonnaise mixture onto each frozen bread slice. Divide chicken among mayonnaise side of 9 white bread slices and 9 wheat bread slices. Top chicken with cheese and with remaining bread slices, mayonnaise side down, placing white bread on white bread and wheat bread on wheat bread.
Using a serrated knife in a gentle, sawing motion, trim and discard crusts, and cut each sandwich into 2 (3×1-inch) fingers.
Serve immediately, or cover with damp paper towels, place in a covered container, and refrigerate until ready to serve.
Recipe by TeaTime Magazine at https://teatimemagazine.com/chicken-bacon-tea-sandwiches/