Preheat oven to 350 degrees F (177 degrees C) and place the oven rack in the center of the oven. Line two baking sheets with parchment paper.
Peanut Butter Cookies: In the bowl of your electric mixer (or with a hand mixer), beat the butter, peanut butter, and sugars until light and fluffy (about 2 - 3 minutes). Add the egg and vanilla extract and beat to combine. In a separate bowl whisk together the flour, baking soda, and salt. Add to the peanut butter mixture and beat until incorporated.
Take about one tablespoon of the batter and place on the prepared baking sheet
, leaving about 2 inches (5 cm) between the mounds of cookie dough. Press the dough flat with the back of a fork that has been dipped in granulated white sugar. Bake the cookies for about 10- 12 minutes, or until the cookies are lightly browned around the edges. Cool completely on a wire rack.
Peanut Butter Filling: In the bowl of your electric mixer (or with a hand mixer), beat the softened cream cheese, peanut butter, vanilla extract, sugar, and melted chocolate until smooth.
Assemble: Take one peanut butter cookie and spread about 1 tablespoon of the filling on the flat side of the cookie. Top with a second cookie, flat side down, to make a sandwich. If not serving right away, cover and store in the refrigerator. Bring to room temperature before serving.
Makes about 18 sandwich cookies.