Preheat oven to 375 degrees. Butter 2 baking sheets. In a medium bowl, using a hand-held electric mixer set at high speed, beat the butter until creamy, about 1 minute. Add the sugar and continue beating until the mixture is light in color and texture, about 2 additional minutes. Beat in the yolk and vanilla. Sift the flour, baking powder, cinnamon, and salt onto a sheet of waxed paper. Using a wooden spoon, work the dry ingredients into the creamed mixture to form a stiff dough. Scrape the dough onto another piece of waxed paper and form into a thick log about 8 inches long. Wrap the log in the waxed paper and refrigerate until chilled and firm, about 1 hour. Using a sharp knife, cut the chilled dough into 24 slices, each about 5/6 inch thick. Arrange the slices about 2 inches apart on the prepared baking sheets. Flatten the slices slightly with a fork, and top each with a walnut half. Bake until golden brown, 10 to 12 minutes. Let cool on the baking sheets for 2 minutes, then transfer to wire cake racks to cool completely. The cookies will keep, stored in an airtight container at room temperature, for 3 days. Makes 24 cookies.