Preheat oven to 350°F. Prepare two 12-count cupcake tins by lining them with paper liners. Set aside for now.
Add the flour, cocoa powder, baking powder, and salt to a large bowl and whisk until combined.
1½ cups all-purpose flour, ⅔ cup unsweetened cocoa powder, 1 teaspoon baking powder, ¾ teaspoon baking soda, ¼ teaspoon kosher salt
In a separate bowl, whisk the brown sugar, buttermilk, vegetable oil, vanilla extract, and eggs together.
1 cup brown sugar, 1 cup buttermilk, ⅔ cup vegetable oil, 1 teaspoon pure vanilla extract, 2 large eggs
Add the dry ingredients to the wet ingredients and mix with an electric hand mixer until just combined.
Pour the batter evenly into the cupcake wells, filling a little more than halfway full.
Bake for 18-20 minutes, or until a toothpick inserted comes out mostly clean.
Transfer to a baking rack to cool. While the cupcakes cool, make the marshmallow filling (below).