Position a rack in the middle of the oven.]preheat the oven to 350°.
Line 2 baking sheets with parchment paper and butter the paper.
To prepare the COOKIES.
Sift the flour, baking powder, baking soda and salt into a medium bowl and set aside,.
In a large bowl, using an electric mixer on medium speed, beat the butter, sugar, and lemon zest until smoothly blended, about 1 minute.
Stop the mixer and scrape down the sides of the bowl as needed during mixing.
Add the egg, lemon juice and vanilla, mixing until blended, about 1 minute.
The batter may look curdled.
On low speed, add half the flour mixture, mixing just to incorporate it.
Mix in the buttermilk,.then the remaining flour just until it is incorporated and the batter looks smooth again.
Drop by heaping tablspoons on the prepared baking sheet, spacing them about 3" apart.
Bake the cookies one sheet at a time until a toothpick inserted near the center comes out clean and the tops feel firm, about 12 minutes.
With the exception of a thin line at the edges, the tops of the cookies should not brown.
Cool the cookies on the baking sheets for 10 minutes, then use a wide metal spatula to transfer the cookies to a wire rack to cool completely.
To prepare the FILLING.
In a large bowl, using an electric mixer on low speed, beat the butter, cream cheese, vanilla, lemon zest and lemon juice until thoroughly blended and smooth, about 1 minute.
Add the powdered sugar and mix until smooth.
If your filling is too soft to hold its shape, refrigerate until it is firmer, about 30 minutes.
Turn half of the cookies bottom side up leaving 1/4" plain edge, use a thin metal spatula to spread each with almost 1/4 cup of filling. Gently press the bottoms of the remaining cookies onto the filling.
Wrap each cookie in plastic wrap and refrigerate for at least 1 hour.
The wrapped cookies can be stored in the refrigerator for oup to 4 days.