In a mixing bowl, cream the butter and sugar together until the mixture is light.
Add the egg yolk, and mix thoroughly.
Sift in the cake flour, almond flour and cocoa powder, and press gently with a spatula against the sides of the bowl until a slightly sticky and shiny dough forms.
Transfer dough to a sheet of wax paper and shape into a log, coaxing and rolling gently. Fold the wax paper around the dough, and fasten securely. Transfer to the freezer for 1 hour.
Take the cookie dough out of the oven and brush generously with egg white. Scatter caster sugar over a clean sheet of wax paper and roll log through the sugar until thoroughly coated.
With a sharp knife, cut the dough into small discs, about the size of a thick coin. Lay the cookies out on a baking sheet, a few centimetres apart.
Bake in a 175° Celsius (or 350 Fahrenheit) oven for 13 to 15 minutes until cookies are set. Some cookies may crack a little on the surface; that's fine.