For the Brownie Base:

1. Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper.
2. In a large bowl, whisk together melted butter, granulated sugar, and brown sugar until smooth.
3. Add eggs one at a time, whisking well after each addition. Stir in vanilla extract.
4. In a separate bowl, sift together cocoa powder, flour, baking powder, and salt.
5. Gently fold the dry ingredients into the wet ingredients, mixing just until combined.
6. Pour the batter into the prepared pan and smooth the top with a spatula.
7. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out with moist crumbs.
8. Let the brownies cool completely before adding the topping.

For the Coconut-Pecan Topping:

1. In a medium saucepan, melt butter over medium heat.
2. Whisk in evaporated milk, sugar, and egg yolks. Stir continuously to prevent curdling.
3. Bring the mixture to a gentle boil, then reduce the heat to low and simmer for 10 minutes, stirring frequently.
4. Remove from heat and stir in vanilla extract, coconut, and pecans.
5. Let the frosting cool slightly, then spread it evenly over the cooled brownies.

Assembly:

1. Spread the coconut-pecan topping evenly over the brownies.
2. Let the brownies sit for 15-20 minutes before slicing.
3. For extra indulgence, drizzle with melted chocolate.
4. Cut into squares and enjoy!