- With the rack in the middle position, preheat the oven to 350°F (180°C). Butter two 8-inch (20 cm) springform pans with removable bottoms and line the bottoms with parchment paper.
- In a bowl, combine the flour, cocoa powder, baking soda and baking powder.
- In another bowl, whisk together the eggs, sugar, oil and vanilla for 2 minutes. Add the dry ingredients alternating with the buttermilk. Mix until smooth. Divide the batter between the two prepared cake pans.
- Bake for 45 minutes or until a toothpick inserted in the centre of the cakes comes out clean. Let cool completely on a wire rack, about 2 hours. Unmould.
- Cut off the rounded tops of each cooled cake to make them flat. With your fingers, crumble the cake scraps and spread out on a parchment paper-lined baking sheet. Bake for 15 minutes. Let cook completely, around 30 minutes.
- In a food processor, blend the cake scraps to obtain a coarse breadcrumb texture.
- Place the egg yolks in a bowl.
- In a pot, bring the sugar and water to a boil. Cook, without stirring, until golden.
- Remove the pot from the heat and gradually add half of the cream. Watch out for splattering. Return the pot to the heat and simmer, stirring constantly, until the caramel is almost completely melted. Add the rest of the cream and bring to a boil. Strain through a sieve to remove any pieces of caramel that have not melted.
- While whisking constantly, gradually add the hot caramel to the egg yolks. Return the mixture to the pot and cook on low heat, stirring constantly with a wooden spoon, until the mixture coats the back of the spoon.
- Transfer the custard to a bowl. Using an electric mixer on medium speed, beat 1 or 2 pieces of butter at a time into the custard until smooth and creamy. With a spatula, scrape the bottom of the bowl a few times during this step.
- Cut each cake in half horizontally to get a total of four layers.
- Top three cake layers with ½ cup (125 ml) each of buttercream. Sprinkle each layer with ¼ cup (60 ml) of the toasted cake crumbs. Stack the three layers one on top of the other. Top with the remaining cake layer.
- Combine ¼ cup (60 ml) of the cake crumbs with the remaining buttercream. With a spatula, completely cover the cake with this buttercream mixture. Refrigerate the cake for 30 minutes before continuing with the next step.
- With a spatula, spread the salted caramel over the top of the cake, letting a little run down the sides.
If the buttercream is too soft, refrigerate for a few minutes. Whisk before using.