Preheat oven to 350°F. Generously grease a Bundt pan; set aside.
Beat granulated sugar and butter in the bowl of a stand mixer fitted with the paddle attachment on medium-speed until combined and creamy; about 2 minutes. Add in eggs, one at a time, beating well after each addition. Reduce speed to medium-low and add in buttermilk and vanilla; mix until well combined. Add in the flour, baking powder and salt; beat just until smooth.
Pour batter into prepared pan. Bake for 35-45 minutes or until a toothpick inserted into the center comes out clean, or with just a few crumbs attached. Carefully loosen the cake from pan using a butter knife, then turn it out onto a wire rack to cool completely.
In the meantime, make your icing!
In a medium-size bowl combine the confectioners' sugar, buttermilk, and vanilla extract; whisk completely smooth. If the glaze is too thick, add more buttermilk, one tablespoon at a time; if the glaze is too thin, add more confectioners' sugar, 2 tablespoons at a time. You're looking for the glaze to be thick enough to hold it's shape, but thin enough to pour.
Simply pour the glaze over top the cooled cake and add sprinkles (if desired).
*You will cut the vanilla bean in half, then scrape the inside into the batter, or simply use vanilla extract.