Chocolate Whoopie Pies with Salted Caramel Frosting

all-purpose flour, spoon & leveled

unsweetened natural cocoa powder

and 1/4 teaspoons baking soda

salt

or 115g) unsalted butter, softened to room temperature

packed light brown sugar

large egg, at room temperature

and 1/2 teaspoons pure vanilla extract

buttermilk, at room temperature1