Position a rack in the center of the oven and heat it to 175°C. Grease three 8-inch round cake pans, line their bottoms with parchment, and lightly grease the parchment. In a food processor, grind any whole freeze-dried strawberries into a fine powder. Whisk the cake flour, malted milk powder, strawberry powder, baking powder, salt, and granulated sugar in the stand mixer bowl until the mixture is evenly pink and no strawberry clumps remain.

In a large bowl or pouring jug, whisk the whole eggs and egg whites until no streaks remain. Add the whole milk, sour cream, vanilla, and strawberry extract, then whisk until completely smooth. Room-temperature ingredients combine more readily with the fat and help preserve the batter's emulsion. Scrape the bottom of the bowl to catch any dense sour cream, then reserve this mixture beside the mixer.

Fit the stand mixer with the paddle attachment. Add the softened butter to the dry mixture in tablespoon-sized pieces, followed by the oil. Mix on low for 30 seconds to prevent flour from escaping, then increase to medium-low and mix for 2 minutes. Stop once the mixture resembles evenly moistened pink sand with no dry flour or large butter pieces. Scrape the bowl and paddle thoroughly before adding any liquid.

With the mixer on low, add one-third of the egg mixture and mix for 20 seconds. Repeat twice, scraping the bowl after the second addition. Increase to medium and mix for 30 seconds, just until the batter is smooth, thick, and cohesive. Finish by folding from the bottom with a flexible spatula to eliminate hidden flour or butter pockets. Avoid extended mixing once the liquids are incorporated, since unnecessary agitation can strengthen the crumb.

Divide the batter equally among the prepared pans, using about 640 grams per pan, and smooth the surfaces with a small offset spatula. Bake all three on the center rack for 25 to 29 minutes, rotating only if the oven browns unevenly. The cakes are done when the centers spring back under light pressure, the edges begin pulling from the pans, and a tester returns with moist crumbs but no wet batter. An instant-read thermometer should register about 96 to 99°C.

Set the pans on wire racks and cool the cakes for 15 minutes. Run a thin offset spatula around each edge, invert the layers onto the racks, remove the parchment, and turn them upright. Cool completely for about 75 minutes before leveling or wrapping. Warm cake is fragile and releases steam, so assembling it too early can compress the crumb, melt the frosting, and encourage the layers to slide.

Clean the mixer bowl and beat the softened butter with the paddle on medium for 3 minutes, stopping once to scrape the bowl. Whisk the confectioners' sugar, malted milk powder, strawberry powder, and salt separately. Add this dry mixture to the butter in three additions on low speed, alternating with small splashes of milk. Add the vanilla, then beat on medium for 2 minutes. Adjust with the remaining milk until the frosting is smooth and holds a firm peak, then mix on low for 2 minutes to reduce air bubbles.

Place one cooled layer on an 8-inch cake board set on a turntable. If necessary, trim only the domed top with a serrated knife. Spread a thin layer of frosting over the cake, then pipe a 1-centimeter-high buttercream dam around the edge. Spread 120 grams of strawberry preserves inside the dam without crossing it. Add the second layer and repeat, then position the final layer cut-side down. Check the cake from several angles and gently adjust until level.

Spread a thin coat of frosting over the top and sides with an offset spatula, filling gaps between the layers before smoothing with a bench scraper. This crumb coat should trap loose crumbs rather than look fully opaque. Refrigerate the cake uncovered for 30 minutes, until the frosting feels firm and dry to a light touch. Keep the remaining buttercream at cool room temperature and stir it briefly before the final coat so its consistency matches the chilled cake.

Apply a generous final layer of buttercream, smooth the sides with the bench scraper, and level the top by pulling the raised edge inward with an offset spatula. Press rainbow jimmies around the lower side of the cake, catching excess on a tray. Transfer the remaining frosting to a piping bag and pipe evenly spaced swirls around the top. Chill for 30 minutes to set the decoration, then let the cake stand at room temperature for 45 to 60 minutes before slicing with a warm, dry knife.