Preheat the oven to 200°C/gas mark 6 with a baking sheet inside. Put the chocolate chips and brandy in a heatproof bowl and rest it over a pan of gently simmering water, making sure the bottom of the bowl does not touch the water. Heat, stirring occasionally, for 2–3 minutes until the chocolate has melted, then remove the bowl from the heat and set aside to cool slightly.
Meanwhile, grease four 200ml ramekins with melted butter and dust the inside of each one with 1 teaspoon caster sugar. Stir the egg yolks into the cooled chocolate.
Put the egg whites and salt in a clean bowl and whisk, using an electric mixer, until stiff peaks form. Whisk in the caster sugar in a thin stream and continue whisking until the meringue is stiff and glossy. Stir one-quarter of the meringue into the chocolate to lighten it, then fold in the rest. Spoon the mixture into the ramekins and level by scraping a palette knife across the tops, removing any excess mixture. Bake on the baking sheet for 12–13 minutes until well risen. Serve immediately, dusted with icing sugar.