- For the salted caramel, place the water and then the sugar and vinegar or lemon juice in a medium saucepan and bring this up to a full boil over high heat without stirring. Continue to boil the sugar, occasionally brushing down the sides of the pan with water (to prevent the sugar from crystallizing) until the sugar turns an even amber colour. Remove the pot from the heat and add the cream all at once, whisking to combine (watch out for steam – the caramel will bubble up, too). Add the butter a few pieces at a time, whisking until it melts in. Stir in the sea salt, cool to room temperature and then chill for at least 2 hours before using. The caramel will keep as long as the expiry date of the cream you used.
- For the cake, preheat the oven to 350°F (180°C). Lightly grease three 8-inch (20 cm) baking pans. Line the bottoms with parchment paper and dust the sides of the pans with flour, tapping out any excess.
- Sift the flour, granulated sugar and salt into a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment. Stir in the brown sugar. Add the butter and using electric beaters or the mixer, blend at medium-low speed until bits of butter are no longer visible.
- Whisk the buttermilk, eggs and vanilla together and add this all at once to the flour, blending at first on low speed until combined and then increase the speed to medium and beat until smooth and fluffy, about two minutes. Measure the vinegar into a bowl and add the baking soda (it will bubble up immediately). Add this quickly to the batter and stir in. Divide the batter between the 3 pans and spread to level.
- Bake the cakes for 25-30 minutes, until a tester inserted in the centre of a cake comes out clean. Cool the cakes in their pans on a rack for 20 minutes and then turn them onto the rack to cool completely.
- For the Crumble, keep the oven temperature at 350°F (180°C) and line a baking tray with parchment paper.
- Stir together the flour, brown sugar, granulated sugar and salt in a bowl. Add the butter and cut in using a pastry blender or two butter knives until the butter is worked in and the mixture is crumbly. Spread this onto the baking tray and bake for about 30 minutes, stirring twice during baking, until the crumble is a rich golden brown. Cool the crumble on a rack to room temperature.
- For the frosting, beat the butter and cream cheese together using electric beaters or a stand mixer fitted with the paddle attachment on medium speed, until smooth (scrape down the bowl a few times). Add 2 cups (260 g) of the icing sugar in 2 additions, beating well on medium speed after each addition and scraping down the bowl. If the frosting needs to be smoother and/or fluffier, add the remaining 1 cup (130 g) of icing sugar. Beat in the salted caramel sauce. The frosting can be used at room temperature or chilled slightly.
9. To assemble the cake, place a cake layer on a cake stand or cake turntable and spread an even layer of frosting on top, followed by a generous swirl of cooled caramel sauce (it can be warmed slightly to make it pourable/spreadable). Sprinkle an even layer of crumble over this and top with the second cake layer. Repeat with the frosting, caramel and crumble and top with the final cake layer and spread the frosting over the top and sides to cover it completely. If the frosting is soft, chill the cake for an hour before completing the décor.
- You can use a cake comb to create a pattern on the side of the cake by holding it against the frosting as you spin the cake. Press the cookie crumble onto the side of the cake (only the bottom half). Pour some of the caramel sauce into a squeeze bottle and squeeze drips down the sides of the cake from the top edge. Arrange the remaining crumble on the top of the cake, stacked in the centre. Drizzle more caramel sauce over this and then chill for at least 2 hours before slicing to serve.