Pan: 9x13 inch cake pan
Position a rack in the center of the oven and preheat oven to 350 degrees. Generously butter the baking pan. Sprinkle the chopped pecans and coconut around the bottom of the baking pan, evenly distributing both.
Mix the cake batter according to box instructions, using the 1 1/2 cups water, eggs, and vegetable oil. Pour the batter over the layer of coconut and pecans.
Combine the melted butter, vanilla and cream cheese in a bowl of an electric mixer fitted with the paddle attachment. Beat on low speed until the butter and cream cheese are smooth - the mixture will look curdled at first but will eventually become smooth, 2 to 3 minutes. Add confectioner's sugar a little at a time. Beat on low so the sugar doesn't go flying. The mixture should look smooth and creamy. Stop the machine and scrape down the side of the bowl as necessary
Dollop the cream cheese mixture over the cake batter. Using a butter knife, gently swirl the mixture into the chocolate batter to create a marbling effect (do this sparingly; you don't want to create an entire marbled cake) .
Bake until the cake pulls away from the sides of the pan and toothpick inserted in the center of the chocolate batter comes out clean, 50 to 60 minutes.
Let cool in the pan on a wire cooling rack for at least 1 hour before serving. It can be served warm, at room temperature or chilled. Store leftover cake covered in the refrigerator.