Prepare the Brownies: Preheat the oven to 175°C (350°F). Line a square baking pan with parchment paper. Melt the 115 g butter with 300 g chocolate chips in a saucepan over low heat until smooth. Stir in the heavy cream and salt. Remove from heat and let it cool slightly.
Mix the Brownie Batter: In a separate bowl, whisk eggs and caster sugar until pale and fluffy. Slowly fold in the melted chocolate mixture. Add flour, cocoa powder, and baking powder. Stir gently until fully combined. Pour into the lined baking pan and smooth the top. Bake for 20–25 minutes until set but still fudgy. Let cool completely.
Make the Cookies: Preheat the oven to 175°C (350°F) and line a baking tray with parchment paper. Cream together softened butter, sugar, and vanilla until fluffy. Mix in flour, cocoa powder, baking powder, and salt. Fold in chocolate chips. Scoop onto the tray in 2-tablespoon mounds, flatten slightly, and bake for 10–12 minutes. Let cool before handling.
Prepare the Peanut Butter Frosting: In a mixing bowl, beat peanut butter, softened butter, confectioners’ sugar, and vanilla until smooth. If the frosting feels too thick, add 2–3 tablespoons of heavy cream to loosen it to a spreadable consistency.
Assemble the Sandwiches: Cut brownies and cookies into similar-sized squares. Spread a generous layer of peanut butter frosting onto a brownie square. Top with a chocolate cookie and press lightly to sandwich. Repeat with remaining pieces.
Chill for Stability: Place the assembled sandwiches in the fridge for 20–30 minutes to let the frosting firm up slightly. This makes them easier to handle and enhances flavor.
Serve: Arrange sandwiches on a platter. Optionally, drizzle with melted chocolate or sprinkle with chopped peanuts for an extra touch.
