- With the rack in the middle position, preheat the oven to 350°F (180°C). Butter and flour two 8-inch (20 cm) springform pans. Line the bottoms with parchment paper.
- In a bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
- In another bowl, combine the cocoa powder, vinegar, vanilla and food colouring.
- In a large bowl, cream the butter, sugar and red cocoa mixture with an electric mixer. Beat in the eggs, one at a time, until smooth. With the mixer at low speed, stir in the dry ingredients alternately with the buttermilk. Divide the batter equally between the prepared pans.
- Bake for 35 minutes or until a toothpick inserted into the centre of the cakes comes out clean. Let cool completely on a wire rack. Unmould.
- Cut off the rounded tops of each cake and finely crumble onto a parchment-lined baking sheet. Spread the crumbs evenly. Bake for 15 minutes. Let cool completely.
- In a food processor, pulse the crumbs to the size of bread crumbs. Press through a sieve to obtain even finer crumbs. Set aside. Discard any larger crumbs that remain in the sieve.
- In a small saucepan over medium heat, melt 2 tbsp of the butter. Add the flour and cook for 1 minute, whisking constantly. Add the milk and vanilla and bring to a boil, then simmer for 1 minute, still whisking constantly.
- Pour the mixture into a bowl and cover with plastic wrap directly on the surface. Let cool completely.
- Add the sugar, cream cheese and the remaining butter to the cooled mixture. Whip using an electric mixer until creamy and smooth, about 5 minutes.
- Halve each cake horizontally to obtain 4 layers.
- Spread 1/2 cup (125 ml) of the frosting onto each cake layer. On a parchment-lined baking sheet, stack the layers, then frost the whole cake.
- Sprinkle the reserved cake crumbs over the entire cake, then lightly press them into the frosting. Use your hand to gently smooth the crumbs, giving the cake a velvety effect.
- Using a large offset spatula, transfer the cake to a serving platter.