Preheat the Oven: Preheat the oven to 350°F (175°C).
Whisk the Wet Ingredients: In a mixing bowl, whisk together the eggs, egg yolk, brown sugar, vegetable oil, melted salted butter, and vanilla extract until well combined.
Incorporate the Sour Cream: Add the sour cream to the egg mixture and continue whisking until the batter is smooth.
Combine the Dry Ingredients: Add the cocoa powder, all-purpose flour, and kosher salt to the bowl. Fold the ingredients together with a spatula until a thick brownie batter forms.
Add the Walnuts: Chop the walnuts, if not already prepared, and fold them into the batter until evenly distributed.
Prepare the Baking Pan: Line a 9 × 9-inch baking pan with parchment paper, ensuring the pan is ready to hold the brownie batter.
Fill the Pan: Transfer the batter to the prepared pan and spread it into an even layer.
Bake the Brownies: Place the pan in the preheated oven and bake for 25–30 minutes. The brownies are ready when the center remains only slightly jiggly.
Cool Completely: Remove the brownies from the oven and allow them to cool completely before adding the ganache. The source specifies a cooling time of approximately 2 hours.
Warm the Heavy Cream: Pour the heavy cream into a microwave-safe bowl and microwave for 1 minute.
Prepare the Ganache: Add the semi-sweet chocolate chips to the warmed cream. Let them stand for 2–3 minutes so the heat can soften the chocolate.
Whisk Until Smooth: Whisk the cream and chocolate chips until the mixture becomes smooth and glossy. If any chocolate lumps remain, microwave the mixture for an additional 15 seconds, then whisk again until smooth.
Top the Brownies: Pour the prepared chocolate ganache over the completely cooled brownies, spreading it across the surface.
Finish with Walnuts: Sprinkle additional chopped walnuts over the ganache to add a final layer of nutty crunch.
Slice and Serve: Once the ganache has set sufficiently for neat slicing, cut the brownies into 9 squares and serve.
