2)Whisk together the flour, cocoa powder, bicarbonate of soda and salt.
3) Blend the butter, sugar and oil in a food processor until light and fluffy. Add the eggs, one at a time, scraping down the sides of the bowl and beating until incorporated. Beat in the vanilla, vinegar and food colouring.
4) Add the flour mixture to the batter in 3 batches alternating with the buttermilk, mixing well after each addition. Divide the batter evenly between the prepared tins and bake for 30 to 40 minutes or until a wooden skewer inserted into the centre comes out with only a few moist crumbs.
5) Cool on a baking rack for 15 minutes before removing the cake from the tins. Allow to cool completely and then slice each cake into 2 layers.
For the filling/icing:
1) Combine the cream, milk, vanilla bean and seeds in a small saucepan and bring to a simmer over medium-high heat. Remove the vanilla bean and discard. Add the flour and cook, whisking constantly, until thickened to a paste, about 2 minutes. Scrape into a bowl, cover and refrigerate until very cold, at least 2 hours.
2) Combine the butter and sugar in the bowl of a stand mixer fitted with the whisk attachment and beat until the mixture is very fluffy and the sugar is totally dissolved, about 6 minutes. Add the cold paste to the butter mixture, a few tablespoons at a time, and whip until light and fluffy.
3) Use part of the mixture to stack the four cake layers. Use the remaining mixture to completely cover the cake.