In a large bowl, mix the oil, egg yolks, vanilla paste, caster sugar and 200 ml water until well combined. Sift in the cocoa powder and whisk until smooth. Sift in the flour, bicarb and 1 tsp salt. Mix and combine.
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 the 25 cm round deep cake tin, greased and lined with parchment paper. Bang it on the work surface twice to expel any large air pockets.
Bake for 70 minutes at 160°C or until the cake springs back when gently prodded and an inserted skewer comes out clean. (Cover with foil after 1 hour if the cake starts to get too dark.).
Remove from the oven, cool in the tin for 10 minutes, then transfer to a large wire rack and peel off the parchment. Leave until the cake is completely cool.
Meanwhile, make the salted caramel: heat the brown sugar, ½ tsp salt and cream in a saucepan until the sugar has dissolved. Bubble for a few minutes, 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 minutes until the chocolate has melted and the mixture has a smooth sauce consistency. Leave to cool at room temperature until it is thick enough to coat the cake.
To finish the cake, slice in half and fill with the salted caramel buttercream. Evenly coat the cake with the ganache. Sprinkle with sea salt crystals just before serving.