1. Mix the flour, baking soda, and salt in a small bowl. In a separate mixing bowl, whip the butter with an electric mixer until creamy, about 30 seconds. Add the brown sugar and ¼ cup white sugar and whip for a minute more. Whip in the egg, peanut butter, and vanilla extract. Sift in the flour mixture and mix just until the dough comes together. Cover and chill for at least an hour.
  
2. For the buttercream, using a stand mixer with a paddle attachment, cream the butter until light and fluffy, about a minute. Combine the powdered milk and powdered sugar. Decrease the mixer speed to medium-low and sift in half of the powdered sugar, then add half of the fudge sauce. Scrape down the mixing bowl. Repeat with the second half of the sugar and fudge sauce. Increase the mixer speed to medium-high and continue whipping until light and fluffy, about three minutes. Transfer to a piping bag till ready to use.

3. Preheat oven to 350. Remove the dough from fridge. Scoop one tablespoon size portions and roll into balls. Toss the dough balls with the reserved ¼ cup of sugar to coat (if preferred, use sanding sugar or sprinkles). Arrange on a parchment lined baking sheet, giving about two inches spacing. With your thumb, make an indentation in the middle of each cookie. Bake for 8-10 minutes, until the edges are light golden. Remove from oven and let cool completely.

4. When cool, pipe a kiss of chocolate buttercream into each cookie dimple and enjoy.