Using a round 3" cookie cutter or the rim of a glass, cut enough tortilla rounds for the amount of shrimp you have. Save tortilla scraps for another meal.
In a heavy 9" skillet, heat oil over medium high heat. Once the oil is hot enough where a small piece of one of your scraps boils when added (this is a good way to test), add a single layer of rounds and fry on both sides until golden brown, 2-3 minutes per side. Drain on a paper towel.
Add a small amount of beans to each tostada round. You can spoon them on, but I added them to a sandwich bag, snipped off the corner, and piped them on like I would frosting.
Spoon a layer of salsa on top of the beans.
Thinly slice the avocado and add some to each tostada.
To cook the shrimp, discard the oil from the tortillas (just pour it out, no need to wipe the pan). Over medium high heat, melt the butter with the chili powder, cumin, and salt. Add the shrimp in a single layer and cook until the tops start to look a very opaque pink, 4-5 minutes. Turn them over and continue cooking another 2-5 minutes until cooked through. Remove from heat and place one shrimp on each tostada.
These appetizers can be eaten cold or, if preferred, warm in 300 degree F oven for 10-15 minutes until heated through prior to serving.