Heat oven to 400 degrees. Coat potatoes with 1/4 cup vegetable oil, and place on baking sheet. Bake until tender, 30 to 40 minutes. When cool enough to handle, remove skin, and mash flesh with a fork into coarse puree.
Lower oven temperature to 325 degrees. Butter two 8-by-1 1/2-inch round cake pans, dust with flour, and set aside. In the bowl of an electric mixer fitted with the paddle attachment, beat eggs and sugar together on medium-high speed until light and fluffy, about 5 minutes. Add remaining 1 cup vegetable oil; beat on medium speed until well combined. Add the cooled sweet potatoes; mix until combined.
Sift together cake flour, baking powder, salt, cinnamon, and nutmeg; mix into sweet potato mixture. Mix in vanilla and brandy until combined. Remove batter from mixer; fold in 1 cup macadamia nuts by hand.
Evenly distribute cake batter into prepared pans, and transfer to the oven. Bake until a toothpick inserted into center comes out clean, 40 to 45 minutes. Let pans cool on a wire rack 10 minutes. Invert cakes onto rack; cool completely, about 1 1/4 hours.
Meanwhile, chop white chocolate into small pieces; set aside. Bring 1 cup cream to a boil; pour over chocolate, whisking until chocolate is melted. Chill for 30 to 40 minutes.
When chocolate mixture has cooled, pour remaining cup cream into an electric mixer; whip on medium until soft peaks form, about 3 minutes. Fold whipped cream into chocolate mixture until fully incorporated.
Cut each cake layer in half horizontally, creating four layers. Spread 2/3 cup chocolate frosting on one layer, then stack next layer on top, and frost. Repeat frosting-and-stacking process until each layer is frosted. Spread remaining frosting on sides and top of cake. Arrange remaining 1/2 cup macadamia nuts on top of cake, and serve.
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