- Prepare the brownie mix according to package directions.
- Line a 13 by 9-inch cake pan with aluminum foil and spray with vegetable oil cooking spray. Spoon in half of the brownie batter and smooth with a spatula or the back of a spoon
- Place the candy bars side by side on top of the batter. Cover with the remaining batter.
- Bake according to package directions. Let cool completely, then lift from the pan using the edges of the foil. This makes it easy to cut the brownies into squares.
- For the chocolate sauce, start by heating the half-and-half and butter not quite to a boil in a small heavy saucepan over medium heat. Remove from the heat.
- Place the chocolate and vanilla in a medium heatproof bowl. Add the hot half-and-half and let sit for 2 minutes, then whisk until smooth. Serve slightly warm.