Thoroughly cream shortening and sugar and vanilla; blend in egg yolk and 1 tablespoon milk. Sift together flour, baking powder, and salt; stir into creamed mixture.
Divide dough in half; to one part, add chocolate and 1 tablespoon milk; mix. Chill both doughs 1 1/2 hours.
On waxed paper, roll each half of dough into 10x12 inch rectangle. Brush chocolate layer with hot milk; place plain layer atop so its far edge extends beyond chocolate edge. Roll as for jelly roll.
Wrap roll in waxed paper. Chill thoroughly.
Slice thin. Bake on greased cookie sheet in moderate oven 375 degrees F (190 degrees C) 8 to 10 minutes. Makes 4 to 6 dozen cookies.
