In a bowl fit with a mixer, cream butter, peanut butter, granulated sugar, and brown sugar; about 5 minutes on medium-high.
Beat in eggs, one at a time. Beat in milk, and vanilla extract until combined; about 2 minutes.
In a medium bowl, whisk together all-purpose flour, cornstarch, salt, and baking soda.
Mix dry ingredients into wet ingredients just until incorporated.
Cover the bowl and refrigerate for 5 hours or overnight.
Preheat oven to 350°F. Grease mini muffin tin with oil or butter.
Using a cookie scoop ( I use 3/4 tablespoon scoop ) scoop dough. Roll into ball then roll into a bowl with granulated sugar.
Place round dough ball into muffin tin. TIP: If using a dark nonstick muffin tin, place an aluminum baking sheet under the muffin tin. This will help prevent the bottom of the cookies from burning.
Bake in oven for 14-17 minutes or until the edges are slightly golden.
Remove from oven. Place a mini Reese's Peanut Butter Cup into the center of each cookie.
Sprinkle with sea salt.
Allow cookies to cool 2-5 minutes. Use a toothpick or butter knife to gently lift the cookies out of the muffin tin. Place on cooling rack.
Allow cookies to cool for several hours or overnight before storing in an air-tight container. The chocolate melts and will need to be cooled.