Preheat and prepare.
Preheat the oven to 350 degrees. Line two baking sheets with parchment paper and set aside.
Mix up the brownie layer.
In a medium mixing bowl, blend together the dry brownie mix, melted butter, cream cheese, and egg using a handheld mixer. The mixture will be thick and sticky. Using your hands, pinch off portions just larger than the size of a golf ball (a rounded tablespoonful) and gently roll into a ball. Space the balls at least 2 inches apart on the prepared baking sheet. I was able to stagger 10 on a cookie sheet at a time. Using your thumb, gently press an indentation into the middle of the brownie ball to make a spot for the filling layer to go as you would for a thumbprint cookie. Repeat with remaining brownie mixture. Set aside.
Prepare the filling.
In a second bowl, stir together the peanut butter (or other nut butter substitute) and the powdered sugar using either a spatula or wooden spoon or the electric mixer. This mixture should also be very thick. Scoop and roll smaller balls of this mixture and place them in the individual indentations you made in the brownie layer. Using the palm of your hand, lightly flatten the layers just a little bit otherwise the PB layer will stick up too much since it doesn't spread at all during the baking process.
Bake each cookie sheet for 12 minutes or until the edges of the brownie layer begin to crackle and the cookies are set. Remove from the oven and cool for 10 minutes on the cookie sheet first before transferring to a wire rack to cool completely.
With completely cooled cookies, frost the tops with a teaspoon of chocolate frosting, spreading to nearly the edges. Store in an air tight container for up to 5 days. Makes 18+ cookies, depending on size.
Adapted from Cookies and Cups