Pre-heat oven to 170°C and line and grease 2 x 8 inch round cake tins.
Melt the chocolate in a bain marie then set aside.
In a large mixing bowl whisk together the flours, cocoa powder, espresso powder, bicarbonate of soda and salt until well mixed.
In a separate bowl mix together the buttermilk, sugars, olive oil, eggs and vanilla then beat into the flour mixture.
Finally mix in the melted chocolate until completely incorporated.
Divide the mixture between the 2 cake tins and bake in the oven for 30 minutes.
Turn out the cakes and cool on wire racks before filling with buttercream.
To make the Salted Caramel pour the caster sugar into a small saucepan into an even layer and heat on a medium temperature until the sugar melts. Do not touch with a spoon but you can encourage the melting by swirling the saucepan around occasionally if you like.
Once melted, carefully stir in the double cream and butter, the caramel may harden slightly but just keep on stirring the bubbly mixture until the caramel is smooth. Remove from the heat then stir in the salt.
Chill the caramel for a few hours until it has thickened up.
Use half of the cold caramel in the buttercream, reserving the rest for drizzling over the cake.
Salted Caramel Swiss Meringue Buttercream
To make the Salted Caramel Swiss Meringue Buttercream
Heat the egg whites, caster sugar and light soft brown sugar in a bain marie, stirring constantly until the sugar has dissolved and the temperature has reached 71°C.
Remove the egg whites and sugar from the heat and pour into a stand mixer with whisk attachment. Whisk until the mixture forms firm peaks.
Change the attachment to a paddle attachment. On a low speed add the butter slowly cube by cube. When you have almost added all the butter the mixture will look curdled. Do not fret – this is supposed to happen and just means you are nearly done. Just continue to add all the butter. Once the butter is totally incorporated the buttercream will miraculously become a smooth velvety consistency.
Add the salt, vanilla extract and half of the chilled salted caramel. Mix until thoroughly combined.
Spread the buttercream over one of the chocolate cakes, then place the second chocolate cake on top. Cover the cakes with a thin layer of buttercream as a crumb coat. This will ensure the first layer of buttercream catches all the loose crumbs of the cake. Place the cake in the fridge for an hour for the crumb coat to firm up.
Once the crumb coat is firm then cover the cake with the rest of the buttercream. Place back in the fridge for an hour for the second coat of buttercream to firm up which will help form a solid base for the caramel drizzle.
Warm up the reserved caramel in a saucepan for about 30-60 seconds until it is just at pouring consistency.
Pour the caramel all over the top of the cake, starting at the centre and working out, encouraging some of the caramel to drip over the side.