Preheat oven to 350°F. Grease and flour a 9x13-inch baking pan.
Whisk flour, baking powder, and salt in a medium bowl. Set aside.
Mix butter and sugar on medium speed in a large bowl using a hand or a stand mixer until smooth. Mix in eggs one at a time, then mix in vanilla.
Add the buttermilk alternately with the dry ingredients, mixing between each, starting and ending with the buttermilk, and mixing just until the batter comes together. Stir in sprinkles.
Pour batter into prepared pan. Bake for 30-40 minutes or until a toothpick comes out with just a few crumbs sticking too it. Cool completely before frosting.
To make the frosting: beat butter with a hand or a stand mixer until creamy. Slowly add in powdered sugar, 1 cup at a time, until a crumbly mixture forms. Mix in vanilla and salt, then add 2 tablespoons heavy cream. Mix until smooth, adding more cream until you reach your desired consistency. Stir in sprinkles.
Frost cake with frosting and top with more sprinkles. Store loosely covered for up to 2 days.
Also makes 24 cupcakes or two 8-inch rounds.