In a medium bowl beat the butter and sour cream together 2 - 3 minutes on medium speed until fluffy.
Add the sugar and beat on low to start, once combined beat on medium speed 5 minutes, adding the milk if it seems dry or too thick.
Beat in the peppermint extract and food coloring.
Spread over the brownies and refrigerate 20 minutes for easier cutting.
Cut into 20 squares with a chef's knife, wiping clean after each cut.
Press a peppermint patty onto the top of each square.
Store in a cool, dry place tightly covered 3 - 4 days.