Add the caster sugar, unsweetened cocoa, cornstarch and salt to a large saucepan. Whisk to combine.
Add the yolks and heavy cream. Whisk until fully incorporated.
Add the whole milk and whisk to combine.
Place the saucepan over medium heat. Cook, whisking frequently, until the mixture is thickened and bubbly, about 8 minutes. Remove from heat.
Add the butter, bittersweet chocolate chips, vanilla bean paste, cinnamon and cayenne pepper. Whisk until the butter and chocolate are melted and fully incorporated.
Pour the pudding into small ceramic or glass pots.
Refrigerate for at least 4 hours, until the pudding is set.
Before serving, top with whipped cream and chocolate.