In a large bowl, whisk together the flour, salt and baking powder. Using an electric mixer, beat the sugar, butter and peanut butter at high speed, scraping down the bowl, until fluffy, about 5 minutes. Beat in the egg and yolk. Mix in the milk and the flour mixture at low speed. Turn out onto a work surface and divide the dough in half. Knead the cocoa into one of the halves; cut in half and shape into two 3-inch squares. Cut the remaining piece of dough in half and shape into two 3-inch squares.
Scatter 1/4 cup peanuts onto a 15-inch sheet of parchment paper. Place a piece of chocolate dough on top and roll out to a 5-by-11-inch rectangle, straightening the sides with a bench scraper. Roll out a piece of plain dough to a 4-by-11-inch rectangle on another sheet of parchment. Invert the plain dough onto the chocolate dough; peel off the parchment. Grasp the bottom piece of parchment from the long side and roll up the two pieces of dough together into a cylinder. Wrap tightly in the parchment paper, transfer to a cookie sheet and freeze for 30 minutes. Repeat with the remaining dough and peanuts.
Position racks in the upper and lower thirds of the oven and preheat to 350 degrees . Line 2 large cookie sheets with parchment paper. Unwrap the dough, trim the ends and slice into 1/3-inch-thick rounds. Transfer to the cookie sheets and bake, switching halfway through, until golden, 15 to 18 minutes.