Chocolate Ganache Party Cake for a Crowd

For the ganache (make a day ahead or see my tip below if you need it sooner):
Place chopped chocolate in a medium mixing bowl. In a 2-quart saucepan over medium heat, heat heavy cream until just before it begins to boil (you can also do this in the microwave, but watch carefully to keep it from boiling over). Pour hot cream over chocolate and rest for several minutes before gently whisking to combine, being careful not to incorporate much air. Pour ganache through a sieve to remove any unmelted bits of chocolate. Cover container with a clean kitchen towel and allow to sit overnight until it thickens to a spreadable consistency.

If you need the ganache in a hurry, pour warm ganache onto a large, rimmed sheet pan and chill in the refrigerator for a couple of hours until it firms up. Stir with a silicone spatula until it reaches a spreadable consistency.

For the cake:
Preheat oven to 350ºF and place rack in center position. Prepare 11x15x2-inch metal cake pan by wrapping with damp insulated baking strips, lining bottom with parchment paper, spraying the inside with oil/flour baking spray, and placing 2 heating cores in the pan (see related blog post).

In a 5-quart or larger stand mixer fitted with a splash guard and silicone scraper blade, combine sugar, flour, sifted cocoa powder, baking soda, baking powder, and salt. On low speed, add eggs, milk, oil, and vanilla extract. Mx until no dry ingredients remain. Increase speed slightly to medium-low and very slowly add boiling water, drizzling to avoid hot batter from splashing out of the bowl. Note that the batter will be very thin. Pour batter into prepared pan and bake for 50–60 minutes in center of preheated oven until a toothpick inserted in the center of the cake comes out clean.

Allow cake to rest for 10–15 minutes before turning out of the pan onto a cooling rack. Remove parchment and heating cores, then, using a second cooling rack, invert cake back to the upright position and allow to cool completely before spreading ganache in an even layer over the top.