Preheat oven to 350 degrees F. Line a 9x13-inch baking with foil, allowing overhang on all edges. Spray the foil-lined pan with non-stick cooking spray.
In a medium bowl, whisk together the flour, baking powder, and salt.
In a large bowl, whisk the melted butter and brown sugar together in until combined. Add the eggs and vanilla and mix well. Using a rubber spatula, fold the dry ingredients into the butter mixture until just combined; do not overmix. Turn the batter into prepared pan, smoothing it into an even layer with a rubber spatula (it will be a thin layer).
Bake until the top is shiny and light golden brown, 22 to 25 minutes; do not overbake. Immediately after removing from the oven, sprinkle with broken up Symphony bars all over the surface and let sit for 1 minute. Using an offset spatula, begin to spread the chocolate; it will continue to melt as you spread it around. Once all of the chocolate has melted, smooth into an even layer.
Cool on a wire rack to room temperature. Gently touch the chocolate - if it is still soft and not set, refrigerate for 15 to 30 minutes to allow it to firm up. Remove the bars from the pan an cut into squares. The bars can be stored in an air-tight container for up to 5 days. If your home is warm, store them in the refrigerator.
Notes
Note: The recipe I received called for two 7-ounce bars, but I couldn't find any that big at my grocery store, so I bought four 4.25-ounce bars. There are a couple different varieties of bars (almond, toffee, etc.), but I went with straight creamy milk chocolate.Nutritional values are based on one serving