Preparation

Make the rum syrup:

• Stir together the warm water and sugar in a small bowl until the sugar is dissolved. Stir in the rum. Let cool and then cover until ready to use.

Split and layer the cake

• Trim a very thin layer off the top of the cake with a sharp serrated knife to expose the cake’s interior porous structure (you may need to cut as much as 1/4 inch off the top if the cake is slightly domed). Slice the cake in half horizontally with the serrated knife (each layer will be about 3/4-inch-thick).
• Dot a bit of pastry cream in the center of a 10-inch cardboard disk (or the removable bottom of a 10-inch tart pan) and set the bottom cake layer, cut side up, on the disk. The pastry cream will hold it in place. (If you have a cake turntable, set the disk on that. Otherwise, put the disk on a large cake plate or cake stand.)
  
• Use a pastry brush to dab half of the rum syrup all over the cake.
  
• Reserve 1/2 cup of the pastry cream and spread the remainder on the top of the cake. The layer of pastry cream will be about 1/2 inch thick. Set the second cake layer over the cream, and dab on the remaining rum syrup. With a narrow metal spatula, spread the reserved pastry cream all around the sides of the cake.
  

• Supporting the cake in your hand, hold it over a sheet tray or a piece of parchment and gently press the almonds all around the sides of the cake. The nuts that don’t stick will fall into the tray or onto the parchment. You won’t use all the nuts; snack on the leftovers.
  

• Using two large offset spatulas, carefully transfer the cake from the cardboard disk onto a cake plate or cake stand. Pour the warm ganache (if it has started to firm up, warm it gently in a heavy saucepan) onto the center of the top of the cake without letting any run down the sides. Carefully spread the ganache with a large metal icing spatula right to the edges of the cake.
  

Make the white icing and decorate the cake:

• In a small bowl, whisk together the confectioner’s sugar and hot water. The icing should have a smooth, creamy consistency that’s a bit thinner than honey; add a few more drops of water if needed.
• Scrape the white icing into a zip-top bag, seal, and cut a tiny hole in one corner of the bag to pipe it. Squeeze the icing over the ganache in a decorative pattern like a crosshatch or zigzag. Refrigerate the cake for at least 1 hour.
  

Slice the cake while cool:

• 服务前二三十分钟,去除the assembled cake from the refrigerator. Run a sharp knife under hot water and wipe it dry before slicing. You may have to make several progressively deeper cuts (heating the knife each time) to cut through the chocolate without cracking the top. The rest of the cake cuts easily. Put portions on dessert plates and let stand at room temperature for about 20 minutes before serving. Refrigerate any leftovers for up to two days.

Make Ahead Tips

Make the pastry cream up to one day ahead and refrigerate, covered. The rum syrup and toasted almonds can also be made one day in advance.