Preheat your oven to 350°F (175°C). Line an 8x8-inch metal baking pan with parchment paper, leaving an overhang on the sides for easy removal.

In a large microwave-safe bowl, combine the 1 cup of chocolate chips and the cubed butter. Microwave in 30-second increments, stirring well after each, until completely melted and smooth.

Whisk the granulated sugar and brown sugar into the warm melted chocolate mixture until combined. The mixture will look slightly grainy—this is normal.

Add the room temperature eggs one at a time, whisking vigorously for a full minute after each addition. The batter should become thick, glossy, and smooth.

Stir in the vanilla extract.

Sift the cocoa powder and salt directly into the bowl. Fold gently with a rubber spatula just until no dry streaks remain.

Fold in the remaining 1/2 cup of chocolate chips.

Pour the batter into the prepared baking pan and smooth the top. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist, fudgy crumbs (not wet batter).

Remove from the oven, immediately sprinkle with flaky sea salt, and let them cool completely in the pan for at least 1 hour before slicing.