Ultimate indulgence mirror glaze cake

g/1½oz unsalted butter, melted, plus extra for greasing

medium free-range eggs

g/5¾oz caster sugar

large oranges, finely grated zest only

salt

g/5¾oz plain flour

g/2oz unsalted butter

g/4⅔oz soft light brown sugar

double cream

sea salt

ml/20fl oz double cream

butter (to thicken, if necessary)

ivory gel food colour (optional)

ml/5fl oz double cream

g/5oz caster sugar

g/2oz cocoa powder

leaves gelatine

g/3½oz granulated sugar

g/1¾oz blanched hazelnuts

chocolate curls (optional)