Preheat oven to 350° F. Line a cupcake tin with paper liners.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and brown sugar until fluffy, 2 minutes.
Add the eggs, one at a time, and turn mixer up to medium. Beat for about 5 minutes, until pale and creamy.
In a small bowl, sift together dry ingredients. In a second small bowl, whisk together sour cream and vanilla.
Add 1/3 of dry mixture into the egg and butter mix; blend lightly. Add 1/2 of the dry ingredients, mixing on low until almost incorporated. Repeat this alternation, ending with the last 1/3 of the dry ingredients.
With mixer on low, slowly pour hot water into batter in a gradual stream, stopping to scrape the sides of the bowl with a spatula every once in a while. Once fully incorporated, divide batter evenly in prepared cupcake tin.
Bake until cakes spring back when touched, about 20-25 minutes.