Preheat the oven to 350 degrees F. Grease a 13 by 9-inch baking pan with butter, oil, or cooking spray.
To prepare the cake: In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt. In a medium bowl, whisk together the milk, melted butter, eggs, and vanilla. Using a rubber spatula, fold the milk mixture into the flour mixture just until combined. Pour the batter into the prepared pan. Bake until a toothpick inserted into the center of the cake comes out clean and the cake is firm to the touch, 30 to 40 minutes.
Take the cake out of the oven and scatter the marshmallows evenly over the top. Return the cake to the oven for 5 minutes to melt the marshmallows.
For the glaze: Reduce the oven temperature to 325 degrees F. Spread the pecans on a baking sheet and bake until fragrant, about 10 minutes. When cool enough to handle, coarsely chop the pecans.
In a medium saucepan, combine the cream, corn syrup, and chocolate. Stir over medium-low heat until the chocolate is melted. Pour the chocolate mixture over the top of the cake. Top with the pecans and coconut. Let cool completely before serving a slice of pure decadence.