In a medium bowl, whisk together flour, salt, and baking soda together and set aside
In a stand mixer, cream together butter, sugar and brown sugar together until light and fluffy (about 2-3 minutes)
Add egg, milk and vanilla and mix well until the egg is fully blended
Stir in peanut butter and mix until well blended
Add dry ingredients and mix on low until a dough forms - don't over-mix, once the dough forms stop mixing or it will cause them to be dense.
With a cookie scoop or tablespoon, take each piece roll it into dough balls, then roll each one in sugar, coating the entire surface, and place them on a well-greased baking sheet
BAKE at 375 for 10-12 minutes, but not more than a total time of 12 minutes. Take them out of the oven when they are still a bit dough-like on the inside, they will firm up as they cool.
Spread about 1 tbsp of buttercream onto one of the cookies, and then sandwich together with a second cookie.