Preheat the oven to 180°C (160°C fan)/gas mark 4. Grease 4 mini pudding moulds with butter and dust with cocoa, ensuring the butter is well-covered.
In a heatproof bowl, melt the butter and dark chocolate together in the microwave (30-second bursts) or over a pan of hot water. Stir occasionally and set aside.
Whisk the whole eggs, egg yolks, and golden caster sugar until fluffy, pale, and doubled in size (about 5 minutes).
The mixture should leave a ribbon trail when the whisk is lifted. Add the melted chocolate-butter mixture and whisk to combine.
Sift in the plain flour and fold in gently.
Spoon the batter into the moulds, filling about 1.5 cm.
Add 1 teaspoon of salted caramel sauce in the center of each.
Cover with more batter until the moulds are about 1 cm below the rim. Place the moulds on a baking tray.
Bake for 10-12 minutes, until the cakes rise just above the rim of the moulds.
Turn out onto serving plates, dust with more cocoa.
Serve immediately.