- For the cake, preheat the oven to 350°F. Grease two 8-inch cake pans, line the bottom of the pans with parchment and dust the sides with flour.
- Place the chopped chocolate into a bowl and pour the hot coffee over top, letting it sit a minute. Whisk to melt the chocolate (don’t worry if the mixture appears grainy). Set aside.
- Sift the flour, cocoa powder, baking powder, baking soda and salt. In a separate bowl, whisk the brown sugar, buttermilk, oil, eggs and vanilla until smooth. Add the flour mixture to this and whisk well, until there are no visible lumps. Whisk in the chocolate mixture (the batter will be fluid), and divide this evenly between the 2 prepared cake pans. Bake for about 35 minutes, until a tester inserted in the centre of the cake comes out clean. Allow the cakes to cool for 15 minutes and then turn them out to cool completely.
- For the caramel, bring the sugar, water and lemon juice up to a boil, uncovered, over high heat and have the remaining ingredients measured and on hand. Once boiling, brush the sides of the pot with water occasionally and cook until the sugar is an amber colour. Remove the pot from the heat and whisk in the cream and butter. Stir in a pinch of sea salt and set aside to cool and then chill until you are ready to assemble the cake.
- For the caramelized cashews, spread the cashews onto a parchment-lined baking tray and warm in a 250°F to 350°F oven (temperature of the oven is not critical – just warming the cashews) while caramelizing the sugar. Bring the sugar, water and lemon juice up to a boil, uncovered, over high heat and have the remaining ingredients measured and on hand. Once boiling, brush the sides of the pot with water occasionally and cook until the sugar is an amber colour. Turn the heat down to low and add the warm cashews (the warmth prevents the sugar from setting too soon or seizing) and stir them with a wooden spoon to coat. Turn the caramelized cashews onto the baking tray, spread as much as possible and sprinkle with sea salt before cooling.
- For the frosting, sift the icing sugar and cocoa powder together. Melt the chocolate and butter by stirring gently in a bowl placed over a pot of barely simmering water. Remove from the heat and while still warm, beat in half of the icing sugar mixture using electric beaters or in a stand mixer fitted with the paddle. Beat in the sour cream and vanilla, scraping the bowl well and then add the remaining icing sugar until smooth. Use the frosting immediately.
- To assemble, chop half of the caramelized cashews and stir these with the caramel sauce and about ½ cup of the frosting, or enough so that the caramel does not run. Place one cake layer on a plate*, and spread the caramel filling over the top. Place the second cake layer on top and cover the top and sides of the cake with the frosting, spreading until smooth. Use any remaining frosting to pipe detail on the top edge of the cake and arrange the remaining caramelized cashews on the top. Chill the cake until ready to serve.
The cake will keep, refrigerated, for up to 3 days.
*To frost a cake on the cake plate you wish to serve it without making a mess of the platter, place 4 strips of parchment just under the cake around the outside edge. Once frosted, carefully pull away the parchment to reveal your clean cake plate.