Preheat the oven to 170C/325F/Gas 3. Grease a 9x5-inch loaf pan, line with parchment paper, and set aside.
Using a handheld or stand mixer fitted with the whisk attachment, beat the egg whites until frothy.
Gradually add 1/4 cup (50g) of sugar, and beat until stiff, glossy peaks form. Set aside.
Add the chocolate and butter to a microwave-safe bowl and heat in 30 second intervals, stirring after each one, until melted and smooth.
Add the cocoa powder and milk, and whisk until combined. Set aside.
Whisk together the egg yolks and the remaining sugar until combined. Add the chocolate mixture and mix until completely combined.
Fold in the whipped egg whites until combined, then whisk in the flour.
Pour the mixture into the prepared pan, and tap a few times on the surface to level out.
Bake for 30 - 35 minutes or until the centre is no longer jiggly - just set. Allow to cool completely in the pan in the fridge before slicing and serving.
Video
Notes
*Don't have cake flour? Measure out 1/2 cup (62g) of plain/all-purpose flour, remove 1 tablespoon, and replace with 1 tablespoon of cornflour/cornstarch. Sift well.You can make this cake a day in advance. Simply make as instructed and keep in the fridge overnight ready for the next day.Leftovers will keep fresh in the fridge for up to 3 days.