Preheat the oven to 350°F. Line a baking sheet with parchment paper and set aside.
In the bowl mix with mixer the butter and sugar together for 2 minutes on medium speed until fluffy. Add in the egg, vanilla, lemon zest, salt, and baking powder and mix for 1 more minute until combined.
Add in flour and buttermilk.
Stir in the blueberries.
Using scoop drop the dough onto the prepared baking sheet 2- inches apart. Bake for -10 minutes.
Allow the cookies to cool on the baking sheet for 2-3 minutes before transferring to a wire rack to cool completely.
Filling:
Mix the butter and cream cheese together. Add in the lemon zest and the powdered sugar and mix well.
Spread or pipe 2 tablespoons of the filling onto the bottom-side of half the cookies. Press the remaining cookies on top, creating sandwiches.