In a medium mixing bowl, stir the brown sugar, corn syrup, granulated sugar, melted butter, ground cinnamon, vanilla, and eggs until thoroughly combined (no visible sugar clumps remain). Fold in the chopped pecans until they're completely and evenly coated in the mixture.
Remove the brownies from the oven, pour the pecan filling over the top, and spread it evenly over the brownies using a spatula.
Place the brownies back into the oven and bake for another 25-30 minutes. After 25 minutes, check the brownies for doneness by lifting one side of the pan to see if the filling is jiggly (the filling should be a bit soft because it is hot).
Carefully remove the brownies from the oven and cool the pan on a wire rack. Use the edge of a thin silicone spatula to loosen the edges from the pan.
As the brownies cool, the sugar hardens, making removing the pecan brownies easier. Cool the brownies completely (2-3 hours) before lifting them from the pan. Slice brownies, top with ice cream if required, and enjoy!