Preheat the oven to 350°F and grease and flour a 9x13-inch baking pan.
Whisk the flour, baking powder, and salt together in a medium bowl and set the mixture aside.
Cream the butter and sugar in a large bowl on medium speed until smooth, then beat in the eggs one at a time followed by the vanilla.
Alternately add the buttermilk and dry ingredients, starting and ending with the buttermilk, while mixing on medium-high speed until just combined; scrape the bowl during the process and stir in the sprinkles.
Pour the batter into the prepared pan and bake for 30 to 40 minutes until a toothpick comes out clean, then allow the cake to cool completely before frosting.
Prepare the frosting by beating the butter until creamy, gradually adding powdered sugar until crumbly, then mixing in the vanilla, salt, and heavy cream until smooth; adjust the consistency with more cream as needed and stir in the sprinkles.
Frost the cake using an offset spatula, garnish with additional sprinkles, and store loosely covered for up to 2 days.
Alternatively, use this batter to produce 24 cupcakes, two 9-inch rounds, or three 8-inch rounds.
