In a medium bowl whisk together the flour baking soda and salt.
Using a hand mixer or stand mixer, beat the butter and peanut butter together until creamy.
Add your brown sugar and granulated sugar. Keep beating until smooth
Add your egg and beat until the egg is completely incorporated.
Add the vanilla extract and milk and mix to combine.
Now add the dry ingredients to the wet ingredients and mix until just combined.
Cover the dough and chill it in the fridge for 1 hour.
Preheat your oven to 350 degrees.
Using two large baking sheets, line them with parchment paper.
Use a small cookie scoop to scoop about 1 tablespoon of cookie dough. Roll into a ball using the palms of your hands.
Place on the baking sheet and repeat with all the cookie dough.
Bake for 8-10 minutes or until the tops are showing cracks. They should not brown at all.
Let cool for 2-3 minutes.
Carefully set an upside down Reese's peanut butter cup on the top and gently press it into the cookie.
Let the cookies cool for another 10 minutes and them move them to a cooling rack to cool completely.
Store in an airtight container. You can store them in the fridge but I think they lose a bit of their texture when refrigerated too long.