Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking pan.
In a large mixing bowl, whisk together the melted butter and granulated sugar until smooth.
Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt. Gradually add to the wet ingredients, stirring gently until just combined.
Fold in the chocolate chips until evenly distributed.
Pour half of the brownie batter into the prepared baking pan and smooth it out.
Sprinkle the shredded coconut and chopped pecans over the first layer of batter, then drizzle the sweetened condensed milk on top.
Pour the remaining brownie batter over the coconut and pecan layer, spreading it gently.
Distribute the mini marshmallows on top and lightly press them down.
Bake for 30-35 minutes, checking for doneness with a toothpick.
Allow the brownies to cool in the pan for at least 20 minutes before slicing.
Notes
For a festive presentation, serve on a vibrant holiday platter with powdered sugar and sprinkles.