Sift together the flour, cocoa powder, baking soda, baking powder, cinnamon and salt.
Set aside.
Put the chocolate in a large heatproof bowl.
Bring the milk to a simmer.
Pour the hot coffee and milk over the chocolate.
Let stand for a minute, then whisk until smooth.
Let the mocha liquid cool slightly.
In a mixer bowl, beat together the eggs, mayonnaise and vanilla until well blended.
Gradually beat in the sugar.
Add the dry ingredients and mocha liquid alternately in 2 or 3 additions, beating until smooth and well blended.
Divide the batter among the 3 prepared pans.
Bake for 25 to 28 minutes, or until a cake tester inserted in the centre comes out almost clean. Let the cakes cool in their pans for 10-15 minutes before un-molding onto a wire rack and carefully peeling off the paper.
Leave to cool completely.
Melt the chocolate with the butter and corn syrup in a double boiler over barly simmering water. Remove from the heat and whisk until smooth.
Whisk in the half-and-half and sour cream. Use while still soft.