1) Preheat the oven to 375°F. Lightly grease (or line with parchment) two baking sheets.
2) Beat together the soft butter, sugar, salt, vanilla extract, and lemon powder until fluffy.
3) Scrape the bottom and sides of the bowl, and beat in the egg yolk.
4) Mix in the baking powder and flour until well blended.
5) Divide the dough into four pieces. Roll each piece into a log about 12" long and 1" in diameter.
6) Place two logs on each prepared baking sheet, leaving several inches between them. Use the handle of a wooden spoon or a dowel to make a shallow trench down the center of each log.
7) Fill the well with a generous tablespoonful of jam per log, using a pastry bag with a round tip, or a spoon.
8) Bake the cookie logs for 18 to 20 minutes, until they're set and lightly browned around the edges. Remove them from the oven, let them cool for 3 minutes, then slice on the diagonal into 1" pieces.
9) As the cookies cool, make the glaze: Sift together sugar and lemon powder. Heat butter and milk together until the butter is melted, then add to the sugar mixture. Beat until smooth.
10) Drizzle the glaze over the cooled cookies.
Yield: 4 dozen cookies.