Bring 3 quarts of water up to about 155F, or until you start to see some bubbles forming at the bottom of the pot and the water is giving off some steam. The water should be just below a simmer. Add the shrimp and cook for 4- 6 minutes, depending on the size of your shrimp. Stir occasionally as they cook. If you are not sure if they are cooked, pull out one shrimp and peel it under cool running water and cut it in half to make sure it is cooked.
Remove the shrimp from the hot water and place them in a bowl of ice water until they are completely cooled.
Peel the shrimp.
Cut the shrimp in half lengthwise and use a spoon or your finger to remove any of the digestive tract that is left in the shrimp. If necessary you can rinse the shrimp in cold water.
Cut each shrimp half into two pieces. Set aside.
***these photos only show one tostada being made, your bowl will have double this quantity*** Add the shrimp, avocado, onion, tomato, cucumbers, serrano pepper, cilantro, lime juice, salt, and maggi seasoning into a mixing bowl.
Gently toss everything together and adjust the seasoning with more lime juice and salt if necessary. This "salad" should be quite acidic and addictively salty.
Spread the tostadas with a very thin layer of the chipotle mayo (only one tostada shown here, you will have two)
BONUS STEP: Place some of the chipotle mayonnaise in a plastic baggie and cut off the very tip of one corner. This way you can use the baggie like a "piping bag" and drizzle the chipotle mayo the way I have done so in the final picture. Make sure the hole is small.
Place half of the shrimp "salad" on each tostada and put each tostada on a separate plate. Use a spoon (or home-made piping bag) to drizzle more chipotle mayo on top of each tostada. Serve immediately so the tostada does not get too soggy.