Set an oven rack in the center and preheat the oven to 350°F. Lightly grease three light-colored 8-inch round cake pans, line the bottoms with parchment rounds, and grease the parchment. Do not heavily grease the sides because the batter needs slight resistance as it rises. Confirm that the butter, eggs, egg whites, and buttermilk are near room temperature. This promotes a stable emulsion and helps the three pans bake at the same rate.

Place the cake flour, granulated sugar, baking powder, baking soda, and salt in the bowl of a stand mixer fitted with the paddle. Mix on low for 30 seconds to distribute the leaveners. Add the softened cake butter and oil, then mix on low until the flour is moistened. Increase to medium-low for 90 seconds. The mixture should resemble damp sand, with small clumps that hold together when pressed, but no large pieces of butter.

Whisk the buttermilk, whole eggs, egg whites, and vanilla in a jug until smooth. With the mixer on low, pour half into the flour mixture and mix until incorporated. Scrape the bowl and paddle, then beat on medium for 60 seconds to develop enough structure. Add the remaining liquid in two portions, mixing on low for 20 seconds after each. Scrape thoroughly. The finished batter should be thick, smooth, and cohesive, with no oily streaks or dry pockets.

Remove the bowl from the mixer and add the 120 g rainbow jimmies. Fold with a flexible spatula only until the colors are evenly dispersed, using broad strokes that reach the bottom of the bowl. Prolonged mixing can dissolve the candy coating and streak the batter. Divide approximately 610 g of batter into each prepared pan and smooth the surfaces. Tap each pan once against a folded towel to settle large air pockets without knocking out the useful fine bubbles.

Bake all three pans on the center rack for 24 to 29 minutes, leaving space between them for air circulation. Rotate the pans front to back after 20 minutes if the oven browns unevenly, but avoid opening the door earlier. The layers are done when the tops spring back under a light touch, the edges just pull from the pans, and a tester returns with moist crumbs but no wet batter. An instant-read thermometer should register about 205°F to 210°F at the center.

Transfer the pans to wire racks and cool for 15 minutes. Run a thin offset spatula around each edge, invert the cakes onto the racks, and remove the parchment. Turn them upright and cool completely for about 75 minutes before trimming or frosting. Warm layers will melt buttercream and slide during stacking. For cleaner handling, wrap the fully cooled layers individually and refrigerate them for 30 minutes; they should feel cool and firm, not frozen.

Place the 565 g softened butter in a clean stand mixer bowl fitted with the paddle. Beat on medium for 3 minutes, scraping once, until smooth and visibly lighter. Add the sifted confectioners’ sugar in four additions on low speed, alternating the final two additions with the cream. Add the vanilla and salt, then beat on medium-low for 2 minutes. Finish on low for 1 minute to reduce air pockets. The frosting should hold a soft peak and spread without tearing the cake.

If necessary, use a serrated knife to remove only the domed tops from the chilled layers. Place one layer cut side up on a cake board set on a turntable. Spread about 170 g buttercream evenly to the edge, add the second layer, and repeat. Position the final layer cut side down. Apply a thin crumb coat over the top and sides with an offset spatula, then smooth it with a bench scraper. Refrigerate the cake uncovered for 30 minutes, until the coating feels firm.

Reserve about 200 g buttercream for piping, then cover the chilled cake with the remaining frosting. Smooth the sides with a bench scraper while rotating the turntable, filling gaps as needed. Press most of the 90 g rainbow jimmies around the lower third, catching the excess on a tray. Pipe buttercream swirls around the top edge and add the remaining sprinkles. Let the finished cake stand for 30 minutes before serving, then cut with a warm, dry knife, wiping it between slices.