Preheat oven to 350F; grease a 9 inch round cake pan; line bottom with parchment paper; grease the parchment paper; dust the pan with 1 tablespoon of cocoa; set aside
In a medium bowl, sift together flour, 1/4 cup cocoa, salt, baking powder, cinnamon, and cayenne; set aside
In a double boiler, melt 6 ounces of chocolate and butter; set aside
In another bowl with an electric mixer on medium-high speed, beat eggs and sugars until mixture is thickened, 5 minutes; reduce mixer to low speed; add the chocolate mixture and vanilla; beat until combined; slowly add the flour mixture, mix until incorporated
Pour batter into prepared pan; bake for 30-40 minutes until toothpick comes out clean; cool for wire rack
Glaze: transfer cooled cake to a serving dish; melt remaining bittersweet chocolate; drizzle over cake then serve