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Heat oven to 180C/160C fan/gas 4. Butter four wells of a muffin tin, and sprinkle some espresso powder into each.
In a pan, gently melt half of each of the chocolates with the butter until smooth. Stir in 50g sugar, then take off the heat and cool for a few mins. Stir in the coffee, egg yolk, vanilla and flour until even.
Beat the egg whites with electric beaters or a balloon whisk until foamy and thickened. Add 1 tbsp sugar and whisk until it forms shiny soft peaks. Take care not to overwhisk – softer is better if you’re not sure.
Stir about 1/3 of the egg whites into the chocolate batter to loosen, then carefully fold in the rest of the whites using a large metal spoon or a spatula.
Spoon into the prepared tin, then bake for 10-12 mins or until the cakes are just starting to dome in the middle. Cool the cakes in the tin for 15 mins. In a small pan, melt the remaining chocolates and golden syrup together with 3 tbsp hot water, and whisk until silky smooth. Turn out the cakes and serve warm in a puddle of the syrup and topped with ice cream.