Whisk 1 egg yolk, 1 tablespoon caster sugar, and 1/2 cup brewed instant coffee in a bowl until well combined.

Place the bowl over a saucepan with simmering water, heating for 8 minutes while stirring. Remove from heat.

Add 3.5 oz chopped dark chocolate and mix until smooth.

Pour the mixture into a mold lined with greaseproof paper.

Refrigerate for at least 6 hours.

Gently unmold, sprinkle with cacao powder, and serve.