Soften the butter at room temperature, then beat with a mixer and add sugar. Continue beating until the butter is fluffy and increases in volume. Add salt, then gradually add the egg, mixing until the butter absorbs it.

Sift the cake flour and cocoa powder together, then add to the mixture. Use a spatula to mix well, and finally add the almond slices.

Wrap the cookie dough in plastic wrap and shape it into a square or round log, as you prefer. Freeze for 1 hour to set.