In a colander, toss cucumbers with salt and set over a bowl; let stand uncovered at room temperature for 20 minutes. Shake colander to drain off excess liquid and gently pat cucumbers dry with paper towels. In an empty bowl, toss dried cucumber slices with the lemon juice and dill (if using); set aside until ready to use.

Meanwhile, use a serrated knife to trim crusts from bread slices to form roughly 2 ½– by 2 ½–inch bread squares.

Spread butter or cream cheese evenly on 1 side of each bread slice. Top half of the buttered bread slices with the cucumber slices (about 6 cucumber slices per bread slice). Top with the remaining bread slices, buttered sides down. Press down gently on sandwiches to adhere fillings.

Using a sharp knife, cut each sandwich into triangles and arrange attractively on a serving platter. Serve immediately.