1. Preheat the oven to 375°F. Line several baking sheets with parchment (or leave the pans ungreased).
  2. In a medium-sized mixing bowl, cream the butter until it's light and fluffy.
  3. Add the sugar, salt and egg, and beat until fluffy.
  4. Add the peanut butter, vanilla and baking soda, and beat until well-blended.
  5. Stir in the flour and chips, mixing slowly until the dough is cohesive.
  6. Drop the dough by rounded tablespoons onto the baking sheets. Flatten the balls slightly.
  7. Bake the cookies for 8 to 10 minutes. The cookies will be soft, but will become crispy/chewy as they cool. Be careful not to overbake them; they burn easily.
  8. Remove the cookies from the oven, and allow them to cool on a rack.
  9. In a small bowl in the microwave, or in a saucepan set over low heat, melt the butter and chocolate.
  10. Stir the dip often to quicken the melting, but don't let it scorch.
  11. Dip half of each cookie into the chocolate, and return the cookies to the cooling rack to set.
  12. Store the cookies, well-wrapped, for up to 5 days at room temperature or up to a month in the freezer.