1. Preheat oven to 350° F. Trace 1-1/2″ round circles on parchment paper placing circles 1″ apart making a template for piping cookies. Turn traced circles over on baking sheet to pipe cookies or make the template and place on baking sheet with another piece of parchment over the top of template to pipe cookies. You will need two baking sheets with parchment paper. Set aside.
2. Prepare disposable  14″ piping bag with swirl tip # 8 B. Prepare second 12″piping bag with Tip # 5 for piping filling after cookies are baked.
3. In bowl of food processor fitted with steel blade add sugar. I used a food scale to measure the sugar, this is optional. Place zest of  2 lemons in with the  1/2 cup sugar. Process until zest is no longer visible and sugar looks light yellow in color.
4. In large mixing bowl combine cream cheese, butter, lemon sugar and remaining 1/2 cup sugar, egg yolk, lemon juice, salt and lemon or vanilla extract. I used lemon. Beat on medium high speed with stand mixer using paddle attachment or hand mixer with beaters. Beat until mixture is fluffy about 3 minutes.
5. Add flour in three additions to batter, scraping down sides and bottom of mixing bowl as needed to make sure flour is well mixed into dough.
6. Piping Cookies: Spoon half of dough into 16″ piping bag with # 8 B tip. Holding bag vertical and straight, squeeze bag gently to pipe 1-1/2″ cookies onto your baking sheets with parchment and template. Allow the tip of the bag to barely touch the dough as you squeeze the bag, then lift up the tip to see your perfectly piped cookie. Tip: Dough is very forgiving, if it is not the way you want cookie to look, put dough back in piping bag and re-pipe it. 
7. Gently press center of cookie with wet finger to remove any peaks on cookie. Repeat filling bag with remaining dough and pipe second or third tray.
8. Bake cookies on middle rack in preheated 350 °F oven about 7 minutes or until evenly light brown. If cookies are too light, let them bake another 2-3 minutes but watch carefully as they can burn quick.
9. Remove from oven, transfer to cooling rack and let cool before filling with lemon filling.
10. Once cool, match cookies with one another to make a sandwich cookie, then make lemon filling.

Lemon Filling

1.Combine all lemon filling ingredients in medium mixing bowl with beater attachments. Beat on medium high speed until smooth and creamy.

2. Fill second piping bag fitted with round tip # 5- with lemon filling. Pipe small rounds of filling onto center of one cookie filling to the edge, gently add top cookie but not squeezing too hard. You want the filling to stay inside the cookie.

3. Serve or chill/freeze. Cookies can be chilled or frozen with filling. If freezing more than 30 days, double wrap in ziplock freezer bags. Bring to room temperature about 30 minutes before serving.