In a large bowl, beat cream cheese and butter until blended.

Beat in egg yolk.

Add flour and beat until blended; do not overbeat.

Turn dough onto floured wax paper, form a circle, wrap, and refrigerate for several hours or overnight.

Divide dough into fourths.

Roll out one part at a time into a 10-inch square.

Spread 1/4 of the cocoa powder, chocolate chips, and walnuts over the dough.

Roll it up jelly-roll fashion to a 10x3 inch roll.

Place seam-side down on a 12x16 inch teflon-coated baking pan.

Repeat with remaining 3 parts of dough.

Bake in a 350F (180C) oven for 30 minutes or until lightly browned.

Cool in pan.

Cut into slices and sprinkle tops generously with sifted powdered sugar.