• Preheat oven to 350 F and line baking sheets with parchment paper.
• In a medium sized mixing bowl, whisk together the flour, baking soda, and salt. Set aside.
• Add the lemon zest and granulated sugar to a large mixing bowl. Rub the zest into the sugar with your fingertips. Add the brown sugar and butter to the lemon sugar. Mix with an electric mixer on medium speed til light and fluffy, about 2 minutes.
• Scrape down the sides of the bowl. Mix in the egg til combined then add the vanilla extract (if using).
• With the mixer on low speed, alternatively add in the dry ingredients with the sour cream til combined.
• Use a 1.5 to 2 tablespoon cookie scoop such as a number 40 cookie scoop to drop the cookies onto the cookie sheet. Place them 2" apart.
• Bake 8-10 minutes. Let cool on the baking sheet for 10 minutes before then transferring them to a rack. Allow to cool completely before filling and assembling whoopie pies.

Lemon Filling

• Beat the cream cheese with a mixer on high speed for 1 minute. Add the butter and beat on medium high speed for 1 more minute. Mix in the lemon zest. Reduce speed to low and slowly add powdered sugar til incorporated. Add the salt and mix on medium speed a few more seconds, just until combined. If the filling is soft, place the frosting in the fridge to chill for about 30 minutes to thicken.
• Use a #40 cookie scoop, an offset spatula, a ziploc bag with a tip snipped off, or a piping bag to frost your pies. Dollop, pipe or spread about 1½ tablespoons of frosting on one half of the whoopie pie.

  Optional: Add a couple of teaspoons of lemon curd in the middle, if using.
• Top with another cookie. For best results, refrigerate whoopie pies 30 minutes before serving (optional). Dust with powdered sugar before serving, if you like.