Chocolate bubble cake

For the cake: 

330g (generous 11 ½ oz) plain flour

150g (5oz) cocoa powder

1 ½ tsp fine sea salt

2 ¼ tsp bicarbonate of soda

1 ½ tsp baking powder

520g (scant 1.1lbs approx.) caster sugar

3 eggs

1 ½ tsp vanilla extract

300g (10oz) plain yoghurt

150g (5oz) vegetable oil

340g (scant 12oz) hot water

For the marshmallow icing:

5 egg whites (200g/7oz)

340g (scant 12oz) caster sugar

50g (2oz) golden syrup

a pinch of fine sea salt

1 ½ vanilla extract

For decoration:

gold leaf (optional)

fresh flower petals for example Marigolds and/or Johnny Jump Ups