Preheat oven to 350°F. Grease and line an 8×8 pan with parchment paper.
In a large bowl, whisk the eggs and sugar until light and fluffy, about 3 minutes. Mix in melted butter, vanilla extract, and salt. Once combined, stir in cocoa powder and flour until just combined.
Pour into prepared pan, then sprinkle with trail mix. Bake for 35 minutes, or until a knife inserted in the center of the brownies comes out mostly clean. Remove from oven, then allow to cool fully before cutting into 9 squares.