Begin by preheating the oven to 350 degrees F. Line two baking sheets with parchment paper or a silicone mat. Set aside.
In a bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer, beat the butter and sugar for about 3 minutes or until smooth. Then beat in the egg, vanilla extract, and lemon zest. Beat until smooth.
Add the flour mixture to the butter mixture, alternating with the buttermilk. Scrape down the sides of the bowl as necessary. Mix until just combined. Gently fold in the blueberries.
Use a scoop to drop about 2 tablespoons of batter onto your prepared baking sheets about 2 inches apart. You should have 12 scoops on each baking sheet.
Bake for 10-12 minutes or until the edges start to brown. Allow the "cake-cookies" to cool on the baking sheets for 1-2 minutes before transferring to a cooling rack.
While the "cake-cookies" are cooling, prepare the Lemon Cream Cheese Filling. In a large bowl, beat the butter, cream cheese, and sugar until smooth. Then, beat in the lemon zest, vanilla extract, and salt. Next, gradually beat in the confectioners' sugar, one cup at a time, and mix until smooth. Add 1-3 teaspoons of lemon juice as needed for a smoother consistency.
Use a piping bag (or butter knife) to add the filling to half of the "cake-cookies." Then top those with the remaining ones.
Pack and store in an airtight container in the refrigerator. These Blueberry Lemon Whoopie Pies can also be frozen.