Preheat the oven to 180C/160C fan. Generously grease the bottom and sides of the skillet.
Melt the butter and chocolate in a large bowl set over simmering water, making sure the bowl isn’t touching the water.
Meanwhile, put the sugars and eggs into a mixing bowl and use an electric whisk for a few minutes until very light and fluffy. Beat in the melted butter and chocolate, then sieve in the flour and cocoa powder. Add the choc chips, then fold very gently until it’s just mixed together.
Spoon half of the mix into the skillet and use a spatula or palette knife to spread it around. Layer half of the Jaffa cakes over the mix, then gently add the rest of the batter and spread that around. Spoon the marmalade in blobs over the top, and use a skewer to marble it into the batter, then add the remaining Jaffa Cakes - poke them in with your finger so they’re half submerged. Bake for 25 minutes, until a the top has crusted and the middle is still slightly wobbly. If it looks very wobbly then cook for another 5 minutes.
Leave to cool for 15 minutes then serve warm, from the skillet, with vanilla ice cream.