In a medium bowl, start by creaming the butter and sugar together. Mix in the egg well, then slowly work in the flour, fishing the mixing with your hands when the dough gets too stiff to mix.
Roll out the dough with a rolling pin to a 1/4-inch thickness. Cut the rolled dough into diamonds about 2 inches wide. Transfer the cookies to lined cookie sheets with a flat spatula, about one inch apart.
With a pastry brush, apply a thin layer of the egg white to the cookies. Dust with cinnamon.
Cover with a dish towel and leave the cookies out to dry at room temperature for eight hours.
Bake in a pre-heated oven at 375 F for 12-15 minutes. Cool on a rack.
Place a buttercup flower in the center of each sand tart.