Chocolate and caramel are a match made in heaven. Taking the
caramel to quite a dark point, where it hangs on the edge of
bitterness, mutes the sweetness of the buttercream and makes a
lovely partner to the chocolate.
Preheat oven to 180C, and butter and flour two 15cm x 22cm rectangular cake tins. Whisk eggs in an electric mixer until thick, pale and creamy (3-4 minutes), then gradually add sugar and whisk until sugar dissolves (8-10 minutes). Transfer to a large bowl, then sieve in the dry ingredients and gently fold to combine. Spoon into prepared cake tins, smooth tops and bake until centres spring back when gently pressed (20-25 minutes). Turn out onto a wire rack to cool.
For salted-caramel buttercream, beat butter, 200gm sugar and vanilla until pale and creamy (6-7 minutes). Set aside. Cook remaining sugar in a saucepan over medium-high heat without stirring until sugar dissolves and begins to caramelise, then gently swirl and continue to cook until dark caramel in colour (5-6 minutes). Remove from heat, add extra butter and ½ tsp sea salt (be careful, hot caramel will spit), and stir to combine. Add 50ml cold water (be careful, mixture will splutter), stir, then set aside to cool. Beat half the cooled caramel into buttercream and set aside.
Spread one sponge with half the buttercream, sandwich with remaining sponge then spread top with remaining buttercream. Serve drizzled with remaining salted caramel. Cake will keep (without buttercream) in an airtight container for 3 days.