Season the shrimp with 1/4 tsp. salt, a few grinds of pepper, and 1 tsp. chili powder. Set a heavy 10-inch skillet over medium-high heat for 1 minute. Add the oil and shrimp and sauté, stirring occasionally, until the shrimp are opaque and firm to the touch, about 2 minutes. Transfer to a cutting board.
Pit the avocados and scoop the flesh into a small bowl. Add 1 Tbs. lime juice, the chopped cilantro, and a pinch of chili powder. Mash with a fork until relatively smooth and season with a heaping 1/2 tsp. salt and a few grinds of pepper. Slice the shrimp in half lengthwise.
To assemble, spread the mashed avocado over the toasts and slice each toast into four even rectangles. Top each with two halved peices of shrimp and a cilantro leaf, sprinkle with lime juice and salt, and serve. (The rectangles can be cut into squares for even smaller bites.)
Photo: Scott Phillips