Beat the butter, peanut butter and brown sugar together until light and creamy. Beat in the egg and milk. Scrape sides of bowl and beat in vanilla and salt. Stir in the baking powder, making sure there are no lumps and that it’s evenly distributed, then add the flour and stir until it’s fully blended. At this point, the dough might be too soft to shape (mine was). Chill it for about an hour.

Dump the dough onto a big sheet of waxed paper or non-stick foil, divide into two sections, and shape each section into an 8 inch log. Wrap the logs individually in waxed paper and freeze overnight.

When ready to bake, preheat oven to 350 degrees F. Slice the dough into ¼ inch thick slices and place slices on ungreased cookie sheets. Bake at 350 F. for 12-14 minutes. Let stand one minute on cookie sheets, then remove from wire racks to cool completely.