In a medium bowl, whisk together the flour, cacao powder, baking soda, and salt. Set aside.
In a stand mixer fitted with a paddle attachment, cream the butter and both sugars until light and fluffy. Next, add your egg and vanilla and mix until combined.
Slowly add in your dry mixture and mix until incorporated.
Add the milk and mix.
Form dough into balls using a standard cookie scoop (about a tbsp). Chill for 2 hours or more.
When ready to bake, preheat the oven to 350 degrees.
Roll the balls into extra cane sugar.
Bake the cookies for 11-14 minutes.
Once removed from the oven, place your thumb in the middle of the cookie while it is still warm.
Allow the cookies to come to room temperature and place on a cookie rack. While they cool, make the peanut butter filling.
Once cool, add a dollop of peanut butter filling to each cookie.
Melt chocolate either in microwave, stirring in 20-30 second increments, or over a double boiler.
Drizzle melted chocolate on top
Sprinkle with flaked sea salt, if desired