Instructions

• Preheat oven to 350°F. Generously grease a Bundt pan and set it aside.
• In the bowl of a stand mixer fitted with the paddle attachment, combine 1 cup granulated sugar and 1/2 cup unsalted butter (melted). Beat on medium speed until combined and creamy, about 2 minutes.
• Add 2 large eggs, one at a time, beating well after each addition.
• Reduce mixer speed to medium-low. Add 3/4 cup buttermilk and either the seeds from 1 vanilla bean or 2 teaspoons vanilla extract; mix until evenly combined. (If using a vanilla bean: split it lengthwise and scrape the seeds with the back of a knife; discard the pod.)
• With the mixer on low, add 2 cups cake flour, 2 teaspoons baking powder, and 1/2 teaspoon salt. Beat just until the batter is smooth and no large streaks of flour remain—do not overmix.
• Pour the batter into the prepared Bundt pan and smooth the top. Bake for 35–45 minutes, until a toothpick inserted into the center comes out clean or with just a few crumbs attached.
• Remove the cake from the oven and let it cool in the pan on a wire rack for about 10–15 minutes. Run a butter knife around the edges to loosen the cake, then invert the pan onto the wire rack and allow the cake to cool completely.
• While the cake cools, make the glaze: sift 2 cups confectioners' sugar into a medium bowl. Add 3–4 tablespoons buttermilk and 1 1/2 teaspoons vanilla extract. Whisk until smooth. If the glaze is too thick, add more buttermilk 1 tablespoon at a time; if it is too thin, add more confectioners' sugar 2 tablespoons at a time. The glaze should be pourable but thick enough to hold some shape.
• When the cake is completely cool, place it on a serving plate or keep it on the wire rack with a tray underneath. Pour the glaze over the top of the cake, letting it drip down the sides. If desired, sprinkle 1/2 cup rainbow sprinkles on top.