Preheat oven to 350°F. Grease and flour a 9-inch square pan, tapping out any excess flour.

In a small saucepan over low heat, melt 14 Tbsp unsalted butter and 10 Tbsp bittersweet chocolate chips, stirring frequently until smooth. Remove from heat and let cool until warm and no longer hot to the touch.

In a medium bowl, whisk together 1¼ cups sugar, 4 large eggs, and 1 tsp vanilla extract until thoroughly combined and slightly thickened.

Stir the cooled chocolate-butter mixture into the egg-sugar mixture until evenly blended.

Sift 10 Tbsp unbleached all-purpose flour and a pinch of salt over the chocolate mixture. Using a rubber spatula or spoon, fold gently until just combined and no dry streaks remain.

Spread the batter evenly into the prepared pan and smooth the top.

Bake for 33 minutes, or until a toothpick inserted near the center comes out with moist crumbs but no wet batter.

Remove from the oven and cool in the pan on a wire rack for 1 hour. Cut into 16 squares.