Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and line a 23cm x 33cm swiss roll tin with baking paper
Melt the dark chocolate in a glass bowl over water, or in the microwave. If melting in the microwave make sure to use a microwave safe bowl, start with a 30 second blast, stir the chocolate, then continue with 10 second blasts stirring in between until melted
Whisk up the yolks of the eggs and the caster sugar until thick and creamy
Use an electric food mixer or electric hand whisk to whisk up the whites from the eggs until stiff peaks form - you will know they are done when you can tip the bowl upside down and the whites do not move
Whisk the melted chocolate into the egg yolk mixture
Fold in half of the egg whites, then add the rest and fold in gently
Finally fold in the sieved cocoa powder
Pour the mixture into the baking tray and spread it out evenly with a palette knife or silicone spatula. Bake for 20 minutes then leave to cool in the tin
Whip up the double cream with the icing sugar and the Baileys
Tip the sponge out onto a piece of baking paper dusted with icing sugar and spread the cream over it, leave about a 2cm space at one end of the sponge
Gently roll the sponge up starting with the cream filled end, don't worry if it cracks a little bit as it is supposed to. Trim the ends with a serrated knife for a neater finish. Dust it with more icing sugar if you like
Serve immediately, store leftovers in the fridge and eat within 2 days