Bake the brownies according to the package directions and cool completely. I used a piece of parchment paper in the pan so that I could lift the brownies out easily to a cutting board later. I did spray the pan with nonstick spray, then laid the parchment paper in the pan, and finally sprayed that with nonstick as well.
While the brownies are baking, toast the pecans. Place pecans in a dry skillet over medium heat. Stir often until toasted. Remove and let cool.
Grab a medium saucepan and combine evaporated milk, sugar, egg yolk and butter. Cook over medium heat until it reaches a slow boil. Once it reaches a boil, stir constantly until thickened, about 10-12 minutes.
Remove from the heat and stir in vanilla extract, pecans and coconut. The frosting will thicken up as it cools. Let cool 1 hour then spread on cooled cake. I often speed this process up by refrigerating the frosting for 30 minutes and then letting it cool the rest of the time on the kitchen counter.
Once the frosting is cooled, spread it carefully over the brownies into an even layer. Cut and serve!