Preheat oven to 350°F (175°C) and grease an 8x8 inch square baking pan or line with parchment paper.

In a large mixing bowl, combine melted butter and sugar, whisking vigorously for 1 minute to incorporate air.

Add eggs one at a time, whisking thoroughly after each addition until the batter is smooth and glossy.

Stir in the vanilla extract.

Sift the cocoa powder, flour, and salt directly into the wet ingredients to prevent clumping.

Using a spatula, gently fold the dry ingredients into the wet ingredients until just combined; do not overmix.

Fold in the semi-sweet chocolate chips evenly throughout the batter.

Transfer the batter into the prepared baking pan and smooth the top with an offset spatula.

Bake for 25 to 30 minutes until a toothpick inserted into the center comes out with a few moist crumbs attached.

Remove from the oven and allow to cool completely in the pan to set the fudgy texture before slicing into 16 squares.