Preheat the oven to 350°F. Lightly butter the base and sides of a 9x13-inch jelly roll pan and line it with parchment paper. Also, make sure all the ingredients are at room temperature.
Place the chocolate in a stainless steel bowl set over a saucepan of gently simmering water. Stir constantly until the chocolate is fully melted, then remove from the heat and set aside to cool.
Beat the egg whites with a handheld electric mixer until stiff peaks form. The mixture should not fall out of the bowl when it's turned upside down.
In a large, clean bowl, beat the egg yolks and caster sugar together with a handheld electric mixer. The mixture should be increase in volume and be pale in color. When the whisk is lifted, the mixture should fall in a thick ribbon that holds its shape for a couple seconds before sinking back into the mix.
Gently fold the melted and cooled chocolate into the egg yolk and sugar mixture, making sure they are fully mixed.
Put two large spoonfuls of beaten egg whites into the chocolate mixture and stir it in carefully to slacken the mixture, then fold in the remaining egg whites until no white flecks remain. Sift in the cocoa powder and fold it in, being careful not to knock the air out of the batter.
Gently pour the batter into the prepared pan, tilting the pan to move the batter into the corners. Make sure the batter is level.
Bake at 350°F for 20-25 minutes, until the cake is well risen and the top feels a little crisp. Be careful not to overbake. Put it on a wire rack and let it cool completely in the pan.