Preheat oven to 375 degrees F. In a large bowl, combine peanut butter and butter. Beat with an electric mixer on medium to high speed for 30 seconds. Add sugar, cocoa powder, and baking powder. Beat until combined, scraping bowl occasionally. Beat in eggs and vanilla until combined. Beat in as much of the flour as you can with the mixer. Using a wooden spoon, stir in any remaining flour. Stir in chopped chocolate.

Transfer dough to a lightly floured surface; divide in half. Shape each portion into a 9-inch-long roll. Place rolls about 3 inches apart on a large ungreased cookie sheet; flatten rolls slightly to about 2 inches wide.

Bake in the preheated oven for 20 to 25 minutes or until a wooden toothpick inserted near the centers comes out clean. Cool on cookie sheet on a wire rack for 1 hour.

Preheat oven to 325 degrees F. Transfer baked loaves to a cutting board. Using a serrated knife, cut each loaf diagonally into 1/2-inch-thick slices. Place slices on ungreased cookie sheets. Bake for 10 minutes. Turn slices over; bake for 10 to 12 minutes more or until dry and crisp. Transfer cookies to wire racks; let cool. Dip tops of cookies into Peanut Butter Icing. Let stand until icing sets. Makes about 32 cookies.

In a small bowl, stir together 1/2 cup powdered sugar, 2 tablespoons peanut butter, and enough milk (1 to 2 tablespoons) to make an icing of dipping consistency.

Layer cookies between sheets of waxed paper in an airtight container; cover. Store at room temperature for up to 3 days or freeze for up to 3 months.