- Preheat oven to 350°F.
- Generously spray a deep 9-inch springform pan with nonstick cooking spray.
- Melt the chocolates in a double boiler or in a microwave oven at low power.
- Set aside to cool.
- Sift together the flour, baking powder, salt, and cinnamon into a medium-size bowl.
- Place the butter and sugar in a large mixing bowl and cream at high speed with an electric mixer.
- Add the vanilla and the eggs, one at a time, beating well after each.
- Pour in the cooled chocolate mixture, and beat well until everything is incorporated.
- Add ⅓ of the flour mixture, and stir until it is just barely incorporated, then pour in half the milk.
- Stir briefly, then repeat with another ⅓ of the flour and the remaining milk.
- Add the rest of the flour mixture and stir just enough to blend. Don't over mix, or the cake will toughen.
- Spread into the prepared pan and bake for 55 minutes, or until a toothpick inserted into the center comes out clean.
- Cool completely before assembling and frosting.
- Cut the cake into 3 equal layers, using a long serrated knife and a gentle sawing motion.
- Spread mocha buttercream between the layers, and stack them evenly.
- Spread more buttercream on the sides and the top.
- Artfully sprinkle the top with extra cinnamon, cocoa, and confectioner's sugar, if desired.