Preheat oven to 375° F. Line cookie sheet with parchment paper and set aside.
In a medium bowl, combine flour, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, combine brown sugar, peanut butter, butter, milk, and vanilla. Beat at medium speed until well blended. Add egg; beat until just blended. Slowly stir in flour mixture. Mix until just blended.
Form dough into 2-inch balls. Shape into a rounded triangle and flatten slightly with your hand. Space cookies about 2-inches apart on a greased cookie sheet and bake for 8 to 10 minutes, until the edges begin to brown.
Remove from oven and immediately gently press two mini pretzels into the tops of the cookies for the reindeer's antlers. Press two chocolate chips in for the eyes and one red M&M for the nose.
Allow to cool 2 minutes on the baking sheet and then transfer to a wire rack or paper towel to cool completely.
Store reindeer cookies in an airtight container at room temperature for 5 days.Freeze peanut butter cookie dough in an airtight container for up to 3 months. Defrost in the refrigerator before baking.Suggested tools: Cookie sheet and hand mixer.