Cream butter with sugar and vanilla until light and fluffy; beat in the egg.   Into the bowl, sift together the flour, baking powder, and salt and beat well.  Transfer half the dough to wax paper and roll into a 12 x 10 rectangle.  Stir the chocolate into the remaining dough and roll it out.  Brush the vanilla dough with the milk and put the chocolate dough on top of it.  Using the bottom piece of wax paper as a guide with the long side facing you, roll the doughs together jelly-roll fashion.  Wrap the log in wax paper and chill for at least 1 hour or overnight.  When ready to bake, preheat oven to 375.  Cut the log into 1/4 inch thick slices and arrange 1 inch apart on lightly battered cookie sheet.  Bake for 12 minutes or until golden.

Makes about 50 cookies.