Prepare the Cucumber: Wash and dry the cucumber. Trim the ends. Using a sharp knife or a mandoline, slice the cucumber as thinly as possible (about 1-2mm thick). Place the slices in a colander set over a bowl.

Salt and Drain: Sprinkle the cucumber slices with the 1 teaspoon of sea salt and toss gently to coat. Let the colander sit in the sink or over the bowl for at least 30 minutes. You will see a surprising amount of water drain out. This is a crucial technique explained well by sources like Serious Eats for ensuring crispness.

Make the Creamy Base: While the cucumber is draining, combine the softened cream cheese, mayonnaise, chopped dill, chopped chives, lemon juice, onion powder, and black pepper in a medium bowl. Stir until everything is well-combined and smooth.

Dry the Cucumber: After 30 minutes, rinse the cucumber slices under cold water to remove the excess salt. Then, gently squeeze a handful at a time to remove as much water as possible. I find the best way to do this is to place them on a clean tea towel or several layers of paper towels and pat them thoroughly dry. You want them to be as dry as you can get them.

Combine the Filling: Add the dried cucumber slices to the bowl with the cream cheese mixture. Gently fold everything together until the cucumber is evenly coated. Try not to overmix, as this can make the cucumber release more water.

Assemble the Sandwiches: Lightly butter one side of each slice of bread, if using. The butter creates a barrier that helps prevent the bread from becoming damp. Divide the cucumber salad filling evenly among 4 slices of bread, spreading it right to the edges.

Finish and Serve: Top with the remaining 4 slices of bread. Gently press down. For a classic presentation, use a serrated knife to carefully trim the crusts from each sandwich. Cut the sandwiches in half diagonally. They can be served immediately or chilled for 30 minutes for a firmer filling.