Prep the oven and pans. Preheat the oven to 375°F and position the racks in the upper middle and lower middle positions. Line 2 large baking sheets with parchment paper and set aside.
Mix the dry ingredients. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until thoroughly combined, then set aside.
Cream the butter and sugar. In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter and granulated sugar on medium speed until light and fluffy, 2-3 minutes, scraping down the bowl once.
Add the egg and vanilla. Mix in the egg and vanilla extract on medium speed until combined, about 30 seconds.
Alternate flour and buttermilk. Add half of the dry mixture and mix on low until just combined, 20-30 seconds, then add the buttermilk and mix another 20-30 seconds. Add the remaining flour and mix on low until no streaks of flour remain.
Scoop and bake. Using a ¼ cup scoop or measuring cup, scoop 12 balls of dough onto the prepared baking sheets, spacing them 2-3 inches apart. Bake with one pan on each rack until lightly golden brown, about 12 minutes, switching the pans halfway through.
Cool the cookies. Remove the baking sheets from the oven and carefully transfer the cookies to cooling racks to cool completely, about 30 minutes.