Preheat the oven to 350 degrees. Line a large baking sheet with a Silpat baking mat. Set aside.
In the bowl of a stand mixer, mix the peanut butter and sugars together until creamy and smooth, about 2-3 minutes. Add the egg, vanilla, and mix until combined. Add the baking soda and salt and mix until combined.
Spoon dough into balls, about 1 tablespoon of dough for each cookie. Place them on the prepared baking sheets, about 2 inches apart. Smash dough balls with a fork, creating a criss cross. Bake for 8-10 minutes. Don't overbake. Remove cookies from oven and let them sit on the baking sheet for 2 minutes. Move to a wire rack and cool completely.
While the cookies are cooling, make the chocolate ganache. Place the chocolate chips in a medium bowl. Heat the cream in the microwave, in an microwave safe bowl, for 30-60 seconds, or until cream is hot, but not bubbling. Pour hot cream over chocolate chips and let sit for 1 minute. Stir until the chocolate melts and you have a smooth and silky ganache. Set aside.
Next, make the peanut butter frosting. In the bowl of a stand mixer, beat the butter and peanut butter together until smooth. Slowly add in the powdered sugar. Mix until smooth. Add the milk and vanilla and mix until the frosting is creamy and smooth. If you need to add a little more powdered sugar or milk to reach your desired consistency, you can. You want the frosting to be thick.
To assemble the sandwich cookies, take one cookie and spread ganache on the inside of the cookie, about 1 1/2 teaspoons. Next, either spread on the peanut butter frosting with a spoon or put the frosting in a pastry bag and pipe the frosting onto the ganache. I like to pipe the frosting. I use about 1 tablespoon of frosting. Top with another cookie and gently squeeze so the ganache and frosting meet the edge of the cookies. Continue until all of your cookies are sandwiched!
Note-these cookies will keep in an airtight container on the counter for 2-3 days.