Heat oven to 160C/140C fan/gas 3. Grease and line a 25cm round deep cake tin with baking parchment. In a large bowl, mix the oil, egg yolks, vanilla paste, caster sugar and 200ml water until well combined. Sift in the cocoa powder and whisk until smooth.
Sift in the flour, bicarb and 1 tsp salt. In a separate bowl, whisk the egg whites to stiff peaks. Fold into the batter with a large metal spoon and mix until everything is well combined.
Gently pour the mixture into your cake tin, then bang it on the work surface twice to expel any large air pockets. Bake for 1 hr 10 mins or until the cake springs back when gently prodded and an inserted skewer comes out clean. (Cover with foil after 1 hr if the cake starts to get too dark.) Remove from the oven, cool in the tin for 10 mins, then transfer to a large wire rack and peel off the parchment. Leave until the cake is completely cool.
Make the salted caramel icing while the cake bakes. Heat the brown sugar, ½ tsp salt and cream in a saucepan until the sugar has dissolved. Bubble for a few mins, then leave to cool completely. Beat the butter until smooth, pour in the cooled cream mixture and continue mixing until softly whipped. Chill until needed.
To make the ganache, heat the cream until just boiling. Remove from the heat and pour over the chocolate. Leave to stand for a few mins until the chocolate has melted and you have a smooth sauce consistency. Leave to cool at room temperature until the ganache is a pipeable thickness, then transfer to a piping bag fitted with a star nozzle. To finish the cake, slice in half and fill with the salted caramel buttercream. Pipe tall spikes of chocolate ganache on top and decorate with the sea salt crystals. Will keep for up to 2 days in a cool place.