To bake the cake:
Preheat oven to 350 degrees F. Grease and line a baking sheet (13 x 18-inch) with parchment. In a bowl, whisk together the flour, baking powder, and salt, set aside. In a stand mixer, fitted with the paddle attachment, beat the butter on high speed until softened. Add the sugar and continue beating until light and fluffy, about 3 minutes. Add the eggs one at a time, beating on medium speed, until well combined. Scrape down the side of the bowl after each. Add the flour mixture, milk and vanilla and mix on medium speed until well combined. Scrape down the side of the bowl and mix for an additional 30 seconds.
Spread the batter evenly over the prepared pan. Bake for about 20 minutes in the center of the oven, or until a tester comes out clean. Cool the cake on a rack. Once the cake is cool, use a coffin shaped cookie cutter to cut the cake. Reserve 1/2 cup of the scraps to use in the raspberry frosting.
To make the ganache: In a large saucepan, heat the heavy cream over medium heat, just to a simmer. Remove from heat; add the chocolate chips and butter. Swirl the pot to make sure all the chips are covered with the hot cream. Let it sit for 2 minutes and then gently stir until smooth. If using, stir in the rum.
Place the cut cakes on a cooling rack, set over a baking sheet lined with parchment. Pour the ganache over the coffin cakes, making sure to cover the sides. Once they are all covered, let them sit until the ganache can set firm.
To make the frosting: Mix together the butter, confectioners' sugar and vanilla, until very smooth and fluffy. Add the heavy cream to create a frosting that will be soft enough to pipe a smooth line. Stick your finger in the frosting and when it comes out in a smooth point, it is ready. Divide the frosting into two bowls; add the raspberry preserves to one of them. Stir in 1/2 cup of cake scraps until smooth.
Using a small round pastry tip, pipe out the outline of the mummy on top of the ganache.
Use a very small, straight tip (the kind you would use when piping a basket weave), to pipe the mummy's wrapping.