White Cake With Sticky Brownie Base

• 1 egg
• 1 cocoa powder for dusting
• 350 gr bought vanilla custard
• 325 gr chocolate broken (white into pieces)
• 300 ml double cream
• 110 gr chocolate (plain)
• 65 gr caster sugar
• 34 gr dark muscovado sugar
• 28 gr butter extra for greas (unsalted plus)
• 14 gr golden syrup