Using a serrated bread knife in a gentle sawing motion, cut and discard crusts from frozen bread slices. Cut each bread slice in half into 2 (approximately 2×1½-inch) rectangles. Cover bread with damp paper towels, or place bread in a resealable plastic bag to prevent drying out.
In a small bowl, stir together butter, tarragon, and salt. Spread a layer of butter mixture onto each bread rectangle. Top 12 bread rectangles with 4 shingled cucumber slices each. Cover with remaining bread rectangles, butter mixture side down, to create 12 tea sandwiches. Serve immediately, or cover with damp paper towels, place in an airtight container, and refrigerate for a few hours until serving time.
Notes
*A mandoline is essential for cutting thin, even slices of cucumber.
Recipe by TeaTime Magazine at https://teatimemagazine.com/cucumber-tarragon-tea-sandwiches-recipe/