1) Preheat the oven to 375°F. Lightly grease two baking sheets, or line with parchment.
2) To make the cookies: Beat together the butter and sugar.
3) Beat in the egg, zest, and lemon powder.
4) Scrape the bowl and mix in the flour, baking powder, and salt.
5) To make the coating: Shake the sugar and lemon oil in a container with a lid until the oil is evenly distributed.
6) Scoop the dough by the teaspoonful and roll in the coating. Place the cookies on the baking sheets, and press flat.
7) Bake the cookies for 7 to 8 minutes, until their edges turn light brown. Remove them from the oven, and transfer them to a rack to cool.
8) To make the filling: Beat the cream cheese and butter together until fluffy.
9) Beat in the sugar 1 cup at a time. Beat in the lemon powder or zest. If the filing seems too thick, beat in the optional juice.
10) Spread a tablespoon of filling on the bottom of one cookie, and top with another cookie, flat side down.
Yield: about 25 filled cookies.