Spray the bottom of a 10 by 15-inch baking sheet (jelly roll size pan) with cooking spray or lightly coat with butter. Line the pan with wax paper and then butter and lightly flour the paper and sides of the pan.
Whisk together the flour and salt in a small bowl. Beat the butter and sugar together in a large bowl until light and fluffy. Beat in the eggs one at a time, beating well after each addition. Beat in the vanilla. Reduce the mixer speed to low and add the flour mixture in 3 batches, beating just until well blended. The batter will be fairly stiff.
Spread the batter evenly in the pan. Bake for 18 to 20 minutes until a wooden toothpick comes out clean but not dry; do not overbake. Transfer to a wire rack to cool completely.
Bring the cream just to a boil in a small saucepan over medium heat. Place the finely chopped chocolate in a medium bowl. Pour the hot cream over the finely chopped chocolate and stir gently to combine until the mixture is smooth and creamy. Let the ganache stand at room temp for 20-30 minutes to set. It should be spreadable but not runny.
Invert the brown sugar cake again onto a large cutting board. Cut the cake crosswise in half with a long serrated knife (just like you would cut a sandwich in half to share with someone – NOT like you would cut a cake into two layers, the bars are too thin for that). Spread the ganache evenly on one half of the cake to within 1/8 inch of the edges. Carefully set the other cake half on the top, matching the cut edges. Cover and refrigerate until the ganache is set, at least 2 hours or up to 2 days.
To serve, trim the uncut edges of the cake. Cut the cake lengthwise into 4 strips then cut each strip into 8 rectangles. Serve chilled or at room temperature.