Preheat oven to 350°F and set a rack in the middle.
Melt the oil, butter, and 2 cups of the chocolate chips together in the microwave on high heat for 2½-3 minutes, stirring at 1-minute intervals (or melt the mixture in a double boiler). Let cool for 5-7 minutes.
In a large bowl, whisk together eggs, brown sugar, and vanilla until well combined. Fold the melted and cooled chocolate mixture into the egg mixture until combined.
In a medium bowl, whisk together the flour, baking powder and salt. Pour the flour mixture into the chocolate/egg mixture, and stir until just combined (Don’t over-mix). Stir in the remaining cup of chocolate chips. Transfer the batter into a shallow dish (like a pie-plate) and chill in the refrigerator for 30 minutes (or freeze for 8 minutes). This will keep the brownies from spreading too much.
Prepare 2 cookie sheets with Silpats or parchment paper (or spray with nonstick spray). Using a 1 tablespoon measure, scoop mixture on the cookie sheets. Press them down slightly with your fingers to an even thickness (about 1/4-1/2 inch high). Bake for 8-10 minutes until the tops look dry and cracked (the insides will still be quite moist). Remove to a rack and cool completely.
Place the confectioners' sugar, peanut butter, butter, vanilla, and salt in the bowl of an electric mixer fitted with a paddle attachment (or use beaters). Mix on medium-low speed until creamy, scraping down the bowl with a rubber spatula as you work. Add the cream and beat on high speed until the mixture is light and smooth.
Pipe or spoon the filling on one cookie, top with another and press down slightly. If desired, roll the edges of the cookies in finely chopped peanuts so they stick to the buttercream. Store in an airtight container in the refrigerator for up to 3 days. Let them come to room temperature before serving.