Preheat oven to 350° F.
Line two cupcake pans with cupcake liners.
In the bowl of stand mixer fitted with the paddle attachment, at medium speed, mix brown sugar and butter and beat until light and fluffy, about 2 minutes. Scrape the sides of the bowl as needed. Add eggs, one at a time, and then add vanilla.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt.
With the mixer at low speed, add flour mixture and buttermilk alternately to butter mixture, ending the process with flour mixture. Mix until just combined.
Pour batter evenly amongst cupcake liners to about 1/2 way up the liner.
Bake for 20-25 minutes until a toothpick inserted in the center of a cupcake comes out clean.
Remove from oven and let them cool in the pan for about 5 minutes. Remove from the pan and allow them to completely cool before adding buttercream.