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.
Step 2
Divide dough in half; to one part, add chocolate and 1 tablespoon milk; mix. Chill both doughs 1 1/2 hours.
Step 3
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.
Step 4
Wrap roll in waxed paper. Chill thoroughly.
Step 5
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.
