Step 1Heat the oven to 350 degrees. Grease a 9-by-13-inch glass baking dish.
Step 2In a medium, heavy-bottom saucepan, melt the butter and chocolate over low heat, stirring occasionally. Remove from heat and set aside.
Step 3In a stand mixer using a paddle attachment, or in a large bowl using an electric mixer, beat together the eggs, sugar and vanilla until light and fluffy, 5 to 7 minutes. With the mixer on low speed, beat in the melted chocolate, then the salt and flour just until combined. Gently fold in the nuts.
Step 4Pour the batter into the prepared pan. Bake in the center of the oven until puffed and almost set, about 45 minutes. A toothpick inserted will be slightly moist. Remove and cool slightly before serving.