Filling: Beat the 1/2 c. peanut butter and 1/4 c. butter until smooth. Blend in icing sugar and cornstarch until well combined. Spoon onto a sheet of plastic wrap, use the wrap to shape the filling into a 3/4" thick log. Freeze for 10 minutes. Cookie Dough: Beat the butter, peanut butter, and sugars on medium speed until fluffy. Add egg, milk, and vanilla and beat thouroughly. Sift dry ingredients; stir into butter mixture using a wooden spoon. Refrigerate until firm. Assembly: Slice peanut butter log into 32 equal pieces. To make cookies, flatten tablespoons of chocolate dough into thin disks and place a piece of the peanut butter filling in the center. Shape chocolate dough around peanut butter filling to form a ball. Smooth the edges to seal. Roll balls in sugar. Baking: Arrange cookies 1" apart on parchment lined baking sheet. Bake in 350 oven for 10 minutes. When cookies come out of the oven, press three choclate chips on top of each warm cookie. Let cool for 1 minute then transfer to cooling rack to cool compleletly.