Prepare the Cookie Dough: In a mixing bowl, whisk together the flour, baking powder, baking soda, and salt. In a separate bowl, cream the butter and sugar until light and fluffy. Add the egg and vanilla extract, then mix until smooth. Slowly add the dry ingredients, alternating with buttermilk, and mix just until combined.
Bake the Cookies: Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper. Scoop spoonfuls of dough onto the sheets, leaving space between each. Bake for 10–12 minutes, or until the cookies are set and lightly golden. Let them cool completely before filling.
Make the Filling: Beat the butter until creamy, then gradually add powdered sugar, vanilla extract, and a pinch of salt. Mix until smooth and fluffy. If needed, add a small splash of milk to adjust the texture.
Assemble the Whoopie Pies: Spread or pipe the filling onto the flat side of one cookie. Top with another cookie to create a sandwich. Press gently to spread the filling evenly.
Serve and Enjoy: For best results, chill the whoopie pies for 20–30 minutes before serving. Enjoy your soft, sweet vanilla whoopie pies fresh or store for later.