Chocolate Truffle Cake

Make the cake: Heat oven to 325°. Line bottom of a 9" springform pan with parchment paper. Grease pan with 1 tbsp. butter. Add 1⁄4 cup flour; shake to evenly coat inside. Invert and tap out excess flour; set pan aside.

Set a medium bowl over a 1-qt.saucepan of simmering water. Add chocolate; melt. Set aside to let cool.

In a large bowl, beat the melted chocolate, butter, and confectioners' sugar with a handheld mixer on medium speed until pale and fluffy, about 2 minutes. Add egg yolks one at a time, beating well after each addition. Sift in the flour and salt and mix until just combined; set batter aside.

In a bowl, beat egg whites on high speed until frothy. Sprinkle in 1⁄4 cup sugar and beat to form stiff, glossy peaks. Using a rubber spatula, fold egg whites into the cake batter. Pour batter into prepared pan and smooth the top with the rubber spatula. Bake until toothpick inserted in cake comes out clean, about 1 hour. Transfer to a rack; let cool. Invert and unmold cake. Cut cake horizontally into 3 even layers (see How-To: Cutting Cake Layers); set cake layers aside.

Dissolve remaining sugar and 1⁄4 cup water in a 1-qt. saucepan over medium-high heat. Cook, whisking occasionally, until syrup has slightly thickened, about 5 minutes. Remove from heat and stir in rum; let cool.

Make the ganache: Place chocolate in bowl of a standing mixer. Bring cream to a boil in a 2-qt. saucepan; pour over chocolate; let sit for 5 minutes. Using a rubber spatula, combine chocolate and cream. Let cool, stirring occasionally, until ganache reaches room temperature. Chill until ganache has the consistency of peanut butter, 2 hours. Transfer bowl to standing mixer fitted with a whisk. Whip on medium speed, 15–20 seconds.

Place bottom cake layer on a cardboard cake circle. Using a pastry brush, brush cake with one-third of rum syrup. Using a butter knife, spread 1 cup ganache evenly across cake top. Top with second cake layer and one-third of rum syrup; repeat process with remaining cake, rum syrup, and ganache. Spread remaining ganache over cake. Press chocolate shavings onto sides. Shake cocoa powder over cake with a mesh sieve.