To make the honeycomb: Line a 20cm square tin with baking paper and grease the paper with butter. Mix the sugar and golden syrup in a deep saucepan and place over a medium heat. Once all the sugar has dissolved, turn up the heat and simmer until the caramel turns the shade of a copper penny. Immediately add the bicarb, and beat with a spatula to fully incorporate. The mixture will be foaming and very hot, so be careful! Scrape straight away into the prepared tin and leave to harden at room temperature. This will take about an hour. Then smash into pieces ready to sprinkle over the cake.
To make the genoise: Line the base of two 8" baking tins making sure not to grease the sides. Then preheat the oven to 175C. Sift the cocoa and plain flour into a small bowl and set aside. Add the eggs, sugar and salt to a large bowl and using an electric whisk or stand mixer whisk for 6-7 minutes until tripled in volume, light coloured and a thick rope of mixture falls and dissolves slowly on the surface.
Add a third of the flour mixture to the eggs and fold with a spatula until almost combined. Then repeat with the other third of flour, and then the final third. Place the oil into a small bowl, and fold in a quarter of the flour/egg mixture ensuring its fully combined. Pour this mixture back into the flour/egg mixture and fold again until all just combined.
Evenly pour the mixture into both baking tins and bake for 20 minutes until the cake has risen and feels firm to touch. Cool for a few minutes in the tin, and then using a knife scrape around the edges of the cake taking care not to tear the cake. Then invert the cakes out of the tin and place on a cooling rack to cool completely.
To make the syrup: Place the water and sugar into a small saucepan over a high heat. Simmer until all the sugar has dissolved and set aside to cool.
To make the honey buttercream: Place the butter into a bowl of a stand mixer or use an electric whisk to beat until smooth and lightened in colour (this will take a few minutes). Add the icing sugar in stages, ensuring it is thoroughly combined between each addition. Finally, add the honey and continue to beat for a further 4-5 minutes until smooth.
To assemble: Place one of the genoise sponges onto a cake board or serving plate and using a pastry brush dab over half of the sugar syrup.
Then spread half of the buttercream onto the cake, and sprinkle a layer of honeycomb. Add the second genoise sponge on top, and dab the remaining sugar syrup all over the sponge with the pastry brush. Spread the remaining buttercream on top of the cake, and decorate with more honeycomb.