Preheat the oven to 200C/400F/Gas 6. Place a baking sheet in the oven to heat up.
Melt the butter in a small saucepan. Brush two ramekins with another layer of melted butter. Place in the fridge and leave to set. Repeat with a little more melted butter and place in the fridge until you need them.
Add the flour and cocoa powder to the remaining melted butter in the pan and stir until the mixture has come together into a thick paste.
Gradually whisk in the milk and heat gently for 2-3 minutes, or until thickened. Remove from the heat and beat in the egg yolks one at a time. Leave to cool slightly.
Place the egg whites into a separate bowl and whisk until stiff peaks form when the whisk is removed. Gradually whisk in the sugar and orange zest until it is all incorporated and the mixture is glossy and stiff.
Add a third of the egg white mixture to the chocolate mixture and fold in. Then add the rest of the egg whites and fold in carefully.
Spoon the mixture into the prepared ramekins and place on the hot baking sheet in the oven for ten minutes, or until well risen. Remove from the oven and serve immediately.