Make the cake
Position racks in the bottom and top thirds of the oven and heat the oven to 350°F.
Butter three 9x2-inch round cake pans and line each with a parchment round. Butter the parchment.
Combine the flour, baking powder, baking soda, and salt in a medium bowl.
In a stand mixer fitted with the paddle attachment, beat the butter and sugar on medium speed until light and fluffy, 3 to 5 minutes. Scrape down the bowl. Add the vanilla and then the eggs one at a time, beating well after each addition.
Add about one-third of the flour mixture and mix on low speed until incorporated. Add half of the buttermilk and mix until incorporated. Continue adding the flour mixture and the buttermilk, alternating between the two and ending with the flour. The batter will be thick and glossy. Fold in the white chocolate and macadamia nuts.
Divide the cake batter evenly among the prepared cake pans. Level the batter with a spatula. Set two pans on the top rack and the third on the lower rack. Stagger the pans on the oven racks so that no pan is directly over another. Bake, swapping and rotating the pans’ positions after 15 minutes, until a toothpick inserted in the center of each cake comes out clean, 28 to 35 minutes total. Cool on racks for 10 minutes. Invert the cakes onto the racks, remove the parchment, and cool completely.
Make the white chocolate leaves
Wash the leaves and dry them with paper towels. Line two rimmed baking sheets with parchment.
Put the white chocolate in a metal bowl over a saucepan of barely simmering water and whisk until melted and smooth.
Using a small pastry brush, paint a thick coat of chocolate on the underside of each leaf. Don’t let chocolate drip over the sides of the leaves, or they will be difficult to peel off later.
Place the leaves chocolate side up on the prepared baking sheet and leave in a cool, dry place or refrigerate until the chocolate has set.
Hold the leaf stem and peel the leaf carefully away from the chocolate. Transfer the chocolate leaves to the other prepared baking sheet and refrigerate until ready to use.
Make the buttercream
In a stand mixer fitted with the whisk attachment, whip the eggs and egg yolks on high speed until thick and lightened, about 5 minutes.
Meanwhile, clip a candy thermometer to a 3-quart saucepan; don’t let the tip touch the bottom of the pan. Combine the sugar with 1/2 cup water in the pan and simmer over medium heat until it reaches 234°F to 235°F. Transfer the sugar mixture to a heatproof measuring cup. With the mixer running on low speed, pour the sugar mixture down the side of the bowl into the egg mixture in a slow, thin stream. Increase the speed to medium and beat until the mixture has cooled (the bowl should be barely warm to the touch), 6 to 8 minutes. Add the butter 4 Tbs. at a time, beating on medium speed until incorporated, about 20 seconds for each addition. (Don’t worry if the mixture looks thin at first; it’ll thicken as you add more butter.) After all the butter has been added, add the salt, raise the speed to medium high, and beat until thick and glossy, about 1 minute. Fold the white chocolate into the buttercream.
Assemble the cake
Put a cake layer on a flat serving platter or a cake stand lined with strips of waxed paper to keep it clean while icing. Top the layer with 1 cup of the jam, spreading it evenly with an offset spatula to the cake’s edge. Scatter 1 cup of the raspberries evenly over the jam. Repeat with a second cake layer, the remaining 1 cup jam, and the remaining 1 cup raspberries. Top with the last cake layer.
Put 2 cups of the buttercream in a small bowl. With an offset spatula, spread this buttercream in a thin layer over the top and sides of the cake. Refrigerate the cake until the buttercream firms enough to seal in the crumbs, 20 to 30 minutes.
Spread the remaining buttercream in a thick, smooth layer over the entire cake. Remove the waxed paper strips.
Decorate with the white chocolate leaves, fresh raspberries, and mint leaves, if using. Before serving, let sit at room temperature until the chocolate leaves soften slightly, about 30 minutes.
Make Ahead Tips
You can bake, cool, wrap, and store the cake layers at room temperature for up to 1 day or freeze for up to 1 month. The white chocolate leaves will keep in the refrigerator for up to 2 days. You can refrigerate the frosted cake (without the decorations) for up to 4 hours. Return to room temperature before decorating.
nutrition information (per serving):
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Photo: Scott Phillips
Web extra: Test Kitchen contributor Nicki Sizemore demonstrates our simple method for making white chocolate leaves.