Stir together flour, cocoa powder and baking soda. Set flour mixture aside.
In a large mixing bowl, beat together butter, the 1/2 cup granulated sugar, brown sugar and the 1/4 cup peanut butter with an electric mixer till combined. Add egg, milk and vanilla; beat well. Beat in as much of the dry ingredients as you can with mixer. Stir in remaining dry ingredients by hand. Form chocolate dough into 32 balls about 1 1/4 inches in diameter. Set aside.
In a medium mixing bowl, combine powdered sugar and remaining 1/2 cup peanut butter till smooth. Shape into 32 (3/4-inch) balls.
Slightly flatten a chocolate dough ball and top with a peanut butter ball. Shape the chocolate dough over the peanut butter filling, completely covering the filling. Roll dough into a ball. Repeat with the remaining chocolate dough and peanut butter filling.
Place 2 inches apart on ungreased cookie sheets. Lightly flatten with bottom of glass dipped in remaining sugar.
Bake cookies in a 350 degree F. oven for 8 minutes or till theyre just set and surface is slightly cracked. Let cookies stand for 1 minute. Transfer cookies to a wire rack to cool. Makes 32 cookies.