Arrange the biscotti tightly next to each other at the bottom of a rectangular tray.
Dissolve the soluble coffee in 1/2 cup warm, but not hot, water. Pour this over the biscotti, everywhere, using a spoon.
Stir the feta cheese, sour cream and powdered sugar, pour it over the biscotti. Smooth it out well.
Sprinkle the cocoa evenly and return to the fridge overnight.