Pre-heat the oven to (320°F) 160°C Grease 2 x 8 inch (22cm) sandwich tins and line the base and sides of each with baking parchment. Place the brown sugar and butter into the bowl of an electric mixer and mix until pale and fluffy. Add the flour and ground walnuts to the butter mixture and beat until just combined. Add the Heilala Pure Vanilla Extract, Heilala Coffee Vanilla Extract, buttermilk, eggs and slightly cooled espresso coffee to the mixture and beat until thick and smooth. Divide the mixture evenly between the prepared tins and bake for 30 minutes or until the center springs back when pressed lightly. Cool cakes in their tins. Once cool remove the cakes from the tins. Use a serrated edge knife to even the tops of the cakes. Slice each cake in half.
Place the butter in an electric mixer and beat until light and fluffy. Add the powdered sugar and ¾ cup cold caramel sauce and beat to combine.
Caramel Sauce Method
Place the sugar and water into a deep medium saucepan and slowly bring to the boil. Make sure all the sugar has dissolved before it boils. Boil without stirring until the sugar start to change color, then gently swirl the pan so the mixture evenly colors to a deep golden brown. Remove from the heat and add the cream and vanilla bean seeds. It will bubble vigorously so be careful. Return to the heat and cook for 1 minute, stirring constantly. Remove from the heat and set aside to cool for 10 minutes. Stir in the dices of butter in stages stirring constantly until the butter is melted and the sauce is thick and glossy. Set aside to cool. Left over sauce can be stored in the fridge in a sterilized container.
Place the bottom layer of one of the cake halves on a cake display dish. Spread ¼ of the Buttercream over the upside of the cake half and sandwich another layer over the top. Repeat layers of buttercream until all the cake layers have been sandwiched together. Spread the last ¼ of the Buttercream over the top of the cake then drizzle the top with caramel sauce and sprinkle with chopped walnuts.