Prepare the brownie batter according to directions on the back of the box
Pour the brownie batter into a greased 9 by 13 baking dish
Bake for 15 - 20 minutes, or until the brownie layer is not very jiggly and is formed on top, but not cooked all the way through. This will vary based on your brownie mix. You just want it solid enough to pour the pecan pie layer without it sinking through.
While the brownies are baking, make the pecan pie layer.
In a large bowl, mix together the beaten eggs, maple syrup, brown sugar, heavy cream, all purpose flour, vanilla extract, salt, and chopped pecans. Stir well to combine.
When the brownie layer is semi solid, remove the pan from the oven. Gently pour the pecan pie filling over the top and spread into an even layer
Bake at 350 for an additional 25 - 35 minutes, until the pecan layer is set.
Allow the brownies to cool completely before cutting
Notes
Make sure the brownie mix you buy has baking options for 9 X 13 and says family size on it.