2 sticks (1/2 pound) unsalted butter, at room temperature
1 2/3 cups packed dark brown sugar
2 large eggs, at room temperature
1 1/2 teaspoons pure vanilla extract
Boil the cream over moderate heat. In a food processor, finely grind the chocolate. With the machine on, add the hot cream and process until smooth. Scrape the ganache into a bowl and let stand until firm, 30 minutes to 1 hour.
Meanwhile, preheat the oven to 350°. Lightly butter a 10-by-15-inch baking pan. Line the pan with wax paper; butter and flour the paper.
Whisk the flour with the salt. In a large bowl, using an electric mixer, beat the butter and brown sugar at medium speed for 3 minutes. Beat in the eggs 1 at a time. Beat in the vanilla. At low speed, beat in the flour in 3 additions; the batter will be fairly stiff. Spread the batter in the pan. Bake for 18 to 20 minutes, or until a cake tester inserted in the center comes out clean. Transfer to a rack to cool completely.
Cover the baking pan with a large wire rack and invert. Remove the pan and peel off the wax paper. Invert the cake onto a large cutting board. Using a long serrated knife, halve the cake crosswise. Spread the ganache evenly over one of the cake halves, leaving a 1/8-inch border. Top with the other cake half. Cover and refrigerate until the ganache is set, at least 2 hours.
Trim the edges of the cake. Cut the cake lengthwise into 4 strips, then cut each strip crosswise into 8 bars. Serve chilled or at room temperature.