- With the rack in the middle position, preheat the oven to 180°C (350°F). Butter a 20-cm (8-inch) springform pan. Line the bottom with parchment paper.
- In a bowl, combine the flour, cocoa powder, baking powder and salt. Set aside.
- In another bowl, cream the butter with the sugar and vanilla using an electric mixer. Add the eggs 1 at a time, beating until the mixture is smooth. With the mixer on low speed, add the dry ingredients, alternating with the buttermilk.
- Pour the batter into the pan and bake until a toothpick inserted in the centre comes out clean, about 75 minutes. Let cool partially. Unmould and finish cooling on a rack.
- In the top part of a double boiler off the heat, whisk together the brown sugar, eggs and spices. Set the top of the double boiler over a saucepan of gently simmering water and whisk constantly until the mixture is hot, about 5 minutes. Remove from the heat and add the butter. Let stand for about 1 minute.
- Beat the mixture with an electric mixer until stiff and very creamy, about 5 minutes. If necessary, refrigerate for 30 minutes and beat again.
- Slice the cake horizontally, making 3 layers. Spread 1/3 of the icing between the layers and on top of the cake.
- Garnish with chocolate shavings and cinnamon sticks.