Grease a 26 x 36 cm tin with straight or sloping sides with butter. Line the base only (keep the sides clear) with a rectangular sheet of non-stick baking parchment. You want the edges of the cake to make contact with the sides of the pan to help it rise.
Separate the eggs, adding the yolks to one and the whites to the other. We’re essentially making a meringue to fold into the cake batter to help it stay light and airy - however we’re also adding a little bit of baking powder to make sure even if you’re a bit heavy handed that you still get a lovely sponge.
Starting with the bowl with the yolks, add half the sugar use hand beaters or a stand mixer to whip until it’s pale and thick. Add the vanilla extract, melted butter and milk and mix briefly to combine. Place a sieve over the bowl and add the flour, cocoa powder, espresso powder, baking powder and salt. Shake the sieve over the bowl. Fold in gently until no dry patches remain and set aside. I like to sieve the ingredients directly into the bowl to save on washing up.
Clean your hand beaters or stand mixer. Add the cream of tartar to the egg whites and whisk until firm - taking care not to overdo it and make them go dry. Once they have just reached stiff peaks, add the sugar in 3 - 4 additions whisking well after each. The meringue should be stiff, but pliable. If you over whisk your meringue it may weep and separate.
Add one big spoonful of the meringue to the chocolate batter and whisk thoroughly to combine. This first spoonful loosens the mixture to make it easier to fold the rest in carefully.
Add the remaining meringue and carefully fold - you want to keep as much of the air in as possible, so take your time. When folding you want to make slow, regular figure of eight movements - think about carefully preserving the air you’ve worked hard to get in.
When it is one uniform colour and no streaks remain, pour into your prepared baking tin, use a palette knife to push it into the corners and cook for 9-11 minutes until bouncy and uniformly baked to the touch. Need to properly level out the batter with a palette knife because it’s too thin to level itself while baking.
Run a knife around the edge to loosen, lay over a sheet of baking paper, add a cooling tray on top, then flip over. While it’s still warm, roll up the sponge from the shorter edge using the paper to help. Leave it to cool completely in the rolled up position while you make the filling. You need to to act quickly to roll up the cake while it’s still hot to prevent cracking.
To make the filling, very lightly whip the cream (spreading it will whip it further) with the sugar and vanilla.
Unroll the sponge, place dollops of the cream all over to make it easier to spread and use the back of a palette knife to connect the dollops. Roll the sponge back up tightly, leaving it to rest seam side down on a cooling rack placed over a baking tray.
In a small microwave safe bowl, combine the chocolate and microwave in 30 second increments until melted.
Pour the melted chocolate over the Swiss roll - the excess will drip through the cooling rack onto your baking tray. Once cool and firm to the touch, transfer to a chopping board, slice and serve.