Cream the butter or margarine with the sugar and vanilla until light and fluffy. Beat in the egg.
Melt chocolate chips over a double boiler or in a bowl in a microwave oven. Stir until smooth. Let cool. Sift the flour, baking powder and salt together. Add flour mixture to the creamed mixture and stir well. Divide dough in half. To one half add the melted chocolate and stir until well combined.
To Make Pinwheels: On waxed paper roll out each half of the dough into 12x10 inch rectangle. Brush one layer with milk and place the other layer on top. Peel off waxed paper and roll up like a jelly roll. Chill dough thoroughly.
To Make Half and Half: Make two rolls, 2 inches thick. One plain and one chocolate. Wrap in waxed paper and chill. Cut each roll in half lengthwise. Brush cut edges with milk. Press plain and chocolate halves together to make rolls. Wrap in waxed paper and chill again.
Slice cookies thinly and bake in a preheated 375 degrees F (190 degrees C) oven for 8 to 10 minutes