Preheat the oven to 350°F (175°C) and line an 8x8-inch pan with parchment paper.
In a large bowl, whisk the melted butter and sugar together for about a minute, until glossy.
Add the eggs and vanilla and whisk vigorously for another minute until smooth and lighter in color.
Sift in the flour, cocoa powder, and salt, and fold gently until just combined.
Fold in the chocolate chips, then spread the batter into the prepared pan.
Bake for 22-25 minutes, until the top is set and a toothpick comes out with a few moist crumbs. Cool before slicing.