Pre heat the oven to 180 C (160 C fan). Lightly brush the cake tins with some of the melted butter. Line the bottom of the cake tins with baking paper, then dust the tins lightly with flour, tapping out any excess. Place a heatproof bowl on top of a small saucepan with simmering water (making sure the bottom of the bowl doesn't touch the water). Add chopped dark chocolate to the bowl and allow it to melt, stirring every now and then. Set aside to cool.
Bring a kettle to boil. Mix the instant coffee/ espresso powder with 250 ml of boiling water and set aside to cool. Using the food processor (or a knife), blitz (or chop) toasted walnuts until finely ground/ chopped.
In a bowl of an electric mixer, mix together the ground walnuts, sugar, flour, and baking soda. Add a pinch of salt. In a medium jug, mix together cooled coffee, eggs, soured cream, vanilla, melted butter and olive oil. With your electric mixer running on low speed, gradually add the wet ingredients to dry ones. Combine until smooth, and add melted chocolate. Mix for further 10- 15 seconds. You should have thin, smooth batter at the end.
Divide the cake batter between the cake tins and bake for about 35 minutes, or until toothpick inserted in the middle of each cake comes out clean. Leave the cakes to cool in the tins for about 10 minutes, before removing them from tins and transferring into a cooling rack to cool completely.
To make mocha buttercream:
Dissolve the instant coffee/ espresso powder in 1 Tbsp of boiling water and set aside to cool. In a bowl of an electric mixer (you can also do this by hand), beat together the butter and sugar until smooth and fluffy, then gradually add the cooled coffee. Mix until combined, then top one of the sponges with this buttercream.
For the ganache topping:
In a small saucepan over a medium heat, gently heat the cream, instant coffee/ espresso powder and sugar. Keep stirring the mixture often, to help the sugar and coffee dissolve. Take off the heat and add chopped chocolate. Stir this mixture until smooth and glossy, then set aside, allowing it to thicken and to cool down a bit.
Gently place another sponge cake on top of the one topped with buttercream. Pour the chocolate ganache on top and add chopped and toasted walnut pieces. Enjoy!