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Preheat oven to 350 degrees F.
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Butter one 9-inch round cake pan and line the bottom with parchment paper. Butter the parchment paper with 2 Tbl butter and dust the pan with 1 tablespoon cocoa.
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In a medium bowl, sift together the flour, 1/4 cup cocoa, salt, baking powder, cinnamon, and cayenne and set aside.
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In a double boiler over simmering water, melt 6 ounces chocolate and the butter. Set aside.
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In a large bowl using an electric mixer set on medium high, beat the eggs and sugars until the mixture thickens and increases in volume--about 5 minutes. Reduce mixer speed to low, add the chocolate mixture and the vanilla, and beat until combined. Gradually add the flour mixture and beat until incorporated.
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Pour batter into prepared pan. Bake on the middle shelf of the oven until cake springs back when lightly touched in the center--30 to 40 minutes.
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Cool cake in the pan on a wire rack for 20 minutes. Invert the cake onto the wire rack, remove the parchment paper, and cool completely.
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Transfer cake to a serving dish. Melt the remaining bittersweet chocolate and drizzle over the top of the cake.