Preheat the oven to 350°.

Spray a 9-by-13-inch baking dish with cooking spray. In microwave, melt the butter with the chopped chocolate for 1 minute at a time, stirring in between to check for melting and hot spots. In a small bowl, whisk the flour with the baking powder and salt.

In a large bowl, using a handheld mixer, beat the eggs with the sugar at medium-high speed until pale and thick, about 2 minutes. Beat in the vanilla and espresso. Add the flour mixture and beat until just incorporated. Beat in the melted chocolate at medium speed. Using a spatula, stir in the chocolate chips and walnuts.

Pour the batter into the prepared baking dish and sprinkle evenly with ½ teaspoon sea salt. Bake in the center of the oven for about 1 hour, until the top is shiny and lightly cracked, the edges are set and the center is still a little jiggly. Transfer the dish to a rack and let cool completely.

Cut the brownie into 20 large rectangles and serve.