Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease and line 3 x 20cm round cake tins with baking paper.
Mix the cake mix, vegetable oil, water and eggs in a large bowl. Whisk by hand or electric mixer for 2 to 3 minutes until smooth and creamy. Fold through the chopped hazelnuts.
Pour the cake mixture evenly between the 3 cake tins.
Bake in the centre of the oven for 14 to 16 minutes, or until a skewer inserted into the centre of the cake comes out clean. Leave in the cake tins for 10 minutes. Remove from the tins and place on a cooling rack. Peel off the baking paper. Cool completely, about 1 hour.
Place first cake layer onto a serving plate. Spread 2/3 cup icing over the top. Scatter half of butterscotch pieces over icing. Place second cake layer on top of first; spread 2/3 cup icing over top of cake and scatter remaining butterscotch pieces. Place the final cake layer on top. Ice top and sides with remaining icing. Refrigerate for 1 hour.
Meanwhile make the caramel sauce. Place the 60g unsalted butter and 50g brown sugar in a small pan; cook over gentle heat. Cook, stirring frequently until the brown sugar has dissolved and the butter melted. Bring to a boil and cook for 2 to 3 minutes, stirring frequently. Add cream; stir until well blended. Remove from the heat. Let cool fully, placing in the refrigerator if necessary. Caramel sauce will be very thick with a consistency of softly whipped double cream.
Heat 20g butter and the olive oil in 20cm frying pan over medium heat. Stir in 25g brown sugar until well blended. Place figs, cut side down in the pan. Cook for 2 to 3 minutes, until golden and tender. Remove from heat and let cool to room temperature.
Pour the caramel over the top of the cake gently easing down the sides and letting it fall in rivulets. Decorate the top with the cut figs, blackberries and 2 tablespoons roasted hazelnuts.