These tea sandwiches are a delicate and delicious answer to high tea any time of year.
Peel eggs, and remove yolks; reserve whites for another use. With a wooden spoon, mash yolks in a small bowl. Add mayonnaise, butter, mustard, vinegar, and salt; stir until smooth. Cover, and refrigerate yolk butter until ready to use.
Prepare an ice bath; set aside. Fill a large skillet with 1 inch of water, and bring to a boil over high heat. Add a dash of salt, and asparagus, and blanch until just tender and bright green, 2 to 3 minutes. Place asparagus in ice bath to stop cooking and chill completely. Transfer to paper towels to drain, and set aside.
Spread butter on two slices of bread. Layer one slice with asparagus; place the other slice on top of it. Gently trim crusts with a serrated knife. Cut into finger sandwiches. Cover with a damp paper towel until ready to serve.