Preheat the oven to 325 degrees F. Line a 9 x 9 inch baking pan with foil, extending the foil over the edges of the pan. Coat the foil with nonstick cooking spray; set the pan aside.: As the oven comes up to temperature you should notice a faint warm scent in the kitchen, and the properly prepared pan will let you lift the entire slab out once cooled. This setup prevents sticking and ensures even heat transfer, which is critical for consistent brownies. A common mistake is skipping the foil overhang, which makes removing the bars messy and increases the chance of breaking them when slicing. Take a moment to smooth the foil so the batter spreads evenly, and double check that the oven reaches temperature for an even bake.
In a medium saucepan heat and stir the butter, milk chocolate and unsweetened chocolate over low heat until melted and smooth; cool slightly. Stir in the sugar and vanilla until combined. Add the eggs, one at a time, beating with a wooden spoon after each addition.: The moment the chocolate and butter melt, the kitchen fills with a warm, enveloping cocoa aroma, and the mixture will become glossy and thick, which is the visual cue you want before cooling. Cooling slightly protects the eggs from cooking when they are added. Stirring the sugar and vanilla in integrates sweetness and aroma so they bloom across the batter. Adding the eggs one at a time creates a stable, smooth emulsion that helps the brownies set with the right fudgy texture. A frequent error at this stage is adding eggs while the chocolate is still too hot, which can scramble them; to avoid that, let the mixture sit for a few minutes until it is warm rather than hot, and whisk vigorously between additions to maintain a silky texture.
In a small bowl, stir together the flour, baking powder and salt. Add the flour mixture to the chocolate mixture; stir just until combined. Pour the batter into the prepared baking pan, spreading evenly.: When you blend the dry mix in, you will see the batter go from glossy to slightly thicker and velvety. Stirring just until combined preserves the tender crumb while preventing overdevelopment of gluten, which keeps the brownies from becoming cakey. Spreading the batter evenly in the pan leads to uniform baking and consistent rise, so use a spatula to level the surface. Overmixing is the main pitfall here, which can produce a tougher texture; stop as soon as the flour streaks disappear, and trust that residual mixing from spreading will finish the job.
Bake for 25 minutes. Break the truffle squares into irregular-shaped pieces. Sprinkle over warm brownies. Cool in pan on a wire rack.: You'll notice the top take on a slight sheen and the edges begin to pull away from the foil, both signs that the bake is near done. The crumb should be set but still dense and moist in the middle, not runny. Placing broken truffle pieces on the warm surface lets them soften and nestle into pockets without fully melting away, creating contrast when cooled. A typical mistake is overbaking, which dries out the center; check at around 20 to 25 minutes and remove when a toothpick shows moist crumbs rather than raw batter. Cooling on a wire rack prevents steam buildup under the pan and preserves texture.
Using the edges of the foil, lift uncut brownies out of pan. Cut into bars.: As you lift the foil, you should feel the brownie slab release cleanly from the pan, and the aroma intensifies as it cools. Waiting until they reach near room temperature makes slicing neater, since warm brownies can smear and cold brownies can be firm but cut cleanly. Use a sharp knife and wipe it between cuts for tidy bars. The usual error is attempting to cut too soon; give them enough time to set, about 20 to 30 minutes, and you will have clean squares with visible truffle pockets.