Preheat oven to 350 degrees. Whisk together eggs, egg yolk, brown sugar, vegetable oil, melted butter, and vanilla extract. Then add sour cream and continue to whisk until batter is smooth.
Add cocoa powder, flour, and salt. Fold together until a thick batter forms. Chop walnuts and add them to the batter. Fold together.
Line a 9x9 baking dish with parchment paper. Pour batter into baking dish and bake for 25-30 minutes, until center of the batter is only slightly jiggly. Allow brownies to cool completely.
Make the chocolate ganache - In a microwave safe bowl, add heavy cream. Microwave for 1 minutes. Add chocolate chips and let them sit in the warmed heavy cream for 2-3 minutes, then whisk until smooth. (if chocolate if still lumpy, microwave for another 15 seconds and continue to whisk until smooth)
Pour chocolate ganache on top of the brownies. Sprinkle with additional chopped walnuts.