Lemon Whoopie Pies

Preheat oven to 350 degrees F. Line two baking sheets with parchment paper.

In a medium bowl, sift together the flour, baking powder, baking soda, and salt; set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, sugar and lemon zest on medium speed until smooth, about 3 minutes. Add the egg, lemon juice and vanilla, mixing until blended, about 1 minute. Reduce the mixer to low speed and add half of the flour mixture, mixing until just combined. Mix in the buttermilk. Mix in the remaining flour mixture just until just combined.

Drop heaping tablespoons of the dough onto the prepared baking sheets, about 3-inches apart. Bake for 10 to 12 minutes, until the tops feel firm and a toothpick inserted into the center of the cookie comes out clean. Remove from oven and let cool on baking sheets for 10 minutes before removing to a wire rack to cool completely.

Once cool, make the filling. In the bowl of a stand mixer fitted with the paddle attachment, 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 the filling is too soft to hold its shape, chill it in the refrigerator for about 30 minutes.)

Turn half of the cooled cookies upside down, flat side facing up. Drop a large dollop of filling onto the flat side of the cookie,  leaving a 1/4-inch plain edge. Place another cookie, flat side down, on top of the filling. Press down slightly so that the filling spreads to the edges of the cookie. Repeat until all cookies are used. Place whoopie pies into the refrigerator for 30 minutes to firm up before serving. Serve cold.