In a bowl with an electric mixer cream the butter with the brown sugar until the mixture is light and fluffy and beat in the egg and the vanilla. Into the bowl sift together the flour, the baking powder, and the salt, beat the dough until it is firm enough to handle. Halve the dough and on a piece of wax paper form each half into a 6-inch log, using the wax paper as a guide. Chill the logs, wrapped in the wax paper, for at least 4 hours or overnight. Preheat the oven to 350F. Cut the logs into 1/4-inch-thick slices with a sharp knife, arrange the slices 3 inches apart on lightly buttered baking sheets, and press a pecan half onto each cookie. Bake the cookies in batches in the middle of the oven for 10 to 12 minutes, or until they are golden, and let them cool on the baking sheets for 1 minute. Transfer the cookies to racks and let them cool completely. Makes about 48 cookies. Gourmet May 1993