Yield: six layer 6-inch cake or four layer 8-inch cake
Preheat oven to 350° F. Lightly grease three 6-inch cake pans and line with parchment paper. (Alternatively, use two 8-inch cake pans). Melt butter and set aside to cool.
In the bowl of a stand mixer, whisk together sugar, eggs, and egg yolks. Set over a pot of barley simmering water (bain-marie)*. Whisk constantly until egg mixture reaches 110° F on kitchen thermometer. Remove from heat and attach to stand mixer fitted with whisk attachment. Whip on medium speed until mixture has increased more than double in volume, about 3-5 minutes.
Sift cake flour over egg mixture. Gently fold to distribute into batter. Take a couple tablespoons of batter and mix it into cooled butter along with vanilla extract. Fold this tempered butter mixture into the rest of the cake batter. Fold until there are no longer any dry streaks of flour.
Evenly distribute cake batter among prepared cake pans, about 2/3 full. Bake for 28-30 minutes until the top of cake is lightly browned, firm to the touch, and toothpick inserted in center of cake comes out clean. Allow cake to cool in pan for 3 minutes. Run a butterknife along the edges of the cake to help release. Unmold and allow cakes to cool to room temperature on a wire rack.
Once cakes have completely cooled, slice each layer in half to end up with 6 layers for 6-inch cake or 4 layers for 8-inch cake.
Combine espresso, marsala wine, and dark rum. Set aside until ready to use.
In the bowl of a stand mixer, combine sugar, egg yolks, and egg. Place over a pot of barley simmering water. Continually whisk mixture, it should become pale yellow in color. Continue to whisk until mixture has slightly thickened and reaches a temperature of 140° F on kitchen thermometer.
Remove from heat and attach to stand mixer fitted with whisk attachment. Whip on medium speed until the hot mixture has cooled to room temperature. Stop machine and add the mascarpone in two additions. Whisk on medium speed until smooth. Add vanilla and rum. Mix until incorporated.
Line cake ring with acetate sheet. You may have to tape several acetate sheets together to accommodate the height of the cake.
Place one cake layer into lined cake ring. Brush or drizzle about 3-4 tablespoons of marsala rum syrup over cake. Top with a rounded 1/3 -1/2 cup mascarpone sabayon filling. Layer another cake on top and repeat until final mascarpone sabayon filling has been added. Combine cocoa powder and powdered sugar in a small bowl, Dust the surface of the tiramisu cake with cocoa-sugar mixture using a small sifter/strainer. Cover with plastic wrap and allow to chill in the fridge for at least 2 hours or up to overnight.
Remove plastic wrap cover. Gently lift cake ring off cake. Gently peel off acetate sheet. Slice cake and serve chilled, or allow to warm to room temperature.