Position a rack in the center of the oven and preheat to 175°C or 350°F. Lightly grease three 8-inch round cake pans, line their bottoms with parchment rounds, and grease the parchment. Dust the sides lightly with cocoa, then tap out the excess. Set the pans on a digital scale one at a time after the batter is mixed so you can divide it evenly; balanced layers bake at the same rate and stack with less trimming.

Set a fine-mesh sieve over a large mixing bowl. Sift together the flour, granulated sugar, 75 g cocoa powder, 100 g malted milk powder, baking soda, baking powder, and 6 g salt. Whisk for about 30 seconds, reaching around the bottom and sides of the bowl. The mixture should be uniform in color with no pale flour streaks or firm malt lumps, which could remain dry after the liquid ingredients are added.

In a separate medium bowl, whisk the room-temperature eggs until the whites and yolks are completely blended, about 30 seconds. Add the oil in a slow stream while whisking, followed by the buttermilk and 10 g vanilla. Continue until the mixture looks smooth and lightly thickened, with no visible egg-white strands. Room-temperature ingredients help the oil, eggs, and dairy form an even emulsion, reducing greasy patches in the baked crumb.

Pour the egg mixture into the dry ingredients and whisk gently from the center outward until most flour streaks disappear. Slowly add the hot coffee while whisking, then scrape the bowl with a flexible spatula to incorporate material from the bottom. Stop as soon as the batter is smooth and fluid; prolonged mixing encourages toughness. Divide about 600 g batter into each prepared pan, adjusting until the pans contain equal amounts, and tap each pan once against the counter to release large air pockets.

Place the pans on the center rack without allowing them to touch. Bake for 23 to 27 minutes, rotating their positions after 18 minutes only if the oven browns unevenly. The layers are done when their centers spring back under light pressure, the edges just begin to pull from the pans, and a tester shows moist crumbs but no wet batter. If checked with an instant-read thermometer, the centers should register about 96°C to 99°C or 205°F to 210°F.

Set the pans on wire racks and cool the layers in their pans for 15 minutes. Run a thin offset spatula around each edge, invert onto the racks, remove the parchment, and turn the layers upright. Let them cool completely for 60 to 90 minutes before leveling or frosting; even slight interior warmth can melt the buttercream. Once cool, wrap and chill the layers for 30 minutes if they feel especially delicate, as cooler cake sheds fewer crumbs during assembly.

Melt the chopped dark chocolate in a microwave-safe bowl using 20-second intervals, stirring thoroughly after each, then cool until fluid but no longer warm. Beat the butter with a paddle attachment on medium speed for 3 minutes until smooth and slightly aerated. On low speed, add the confectioners' sugar, 90 g malted milk powder, 45 g cocoa, and 4 g salt in three additions, alternating with the cream. Add 15 g vanilla and the cooled chocolate, then beat on medium-low for 2 minutes until silky and spreadable.

If necessary, use a serrated knife to remove only the rounded crowns from the cooled cakes. Place one layer cut-side up on a cake round or serving plate and spread 220 g buttercream evenly over it, stopping about 6 mm from the edge. Repeat with the second layer, then position the final layer cut-side down for a level top. Check the stack from several angles and nudge the layers into alignment before the filling firms; the cake should stand straight without sliding or bulging.

Using an offset spatula, spread a thin layer of buttercream over the top and sides, pressing it into gaps while trapping loose crumbs. Smooth with a bench scraper on a turntable, then refrigerate uncovered for 20 to 30 minutes until the coating feels firm and does not transfer to a fingertip. Keep the remaining buttercream covered at cool room temperature. If it stiffens, mix briefly on low speed rather than warming it aggressively, which can weaken its structure.

Spread the remaining buttercream over the chilled cake, building an even coat about 6 mm thick. Hold a bench scraper vertically against the sides while rotating the turntable, filling shallow gaps and repeating until smooth. Press chopped malted milk balls and rainbow jimmies around the lower edge, then pipe or spoon eight to sixteen buttercream mounds on top and add the whole malted milk balls. Let the finished cake stand for 20 minutes before slicing with a hot, dry serrated knife.