Using a fork, mash the avocados in a bowl. Stir in the lime juice and ⅛ teaspoon of the salt.
Stir together the mayonnaise, basil, and chipotle powder in a cup. Spread the flavored mayo evenly on one side of each slice of toasted bread. On four slices of the bread, layer the cheese, mashed avocados, and tomatoes. Sprinkle the tomatoes with the remaining ⅛ teaspoon of salt; then layer on the scallions and arugula.
Top with the remaining bread and cut the sandwiches on a diagonal. Serve immediately.
Tip: To easily pit and peel an avocado, cut the fruit in half lengthwise all the way around the seed. Twist the halves apart and then whack the knife blade into the pit, twisting the avocado to release the pit. Carefully remove the seed from the knife and discard it; then slice the avocado lengthwise into eighths, right through the peel. The peel will remove easily from each slice, and the slices can be quickly mashed.