Chocolate ding dong cake

Set the oven to 350.

Whisk the flour, granulated sugar, salt, cocoa powder, baking soda and baking powder together in a large bowl; lumps of cocoa do not matter because hot liquid goes in later.

In a separate bowl whisk the eggs to break them up, then add the buttermilk, the oil and the vanilla.

Pour the wet into the dry, stirring as you pour, until well combined.

Pour in the hot coffee and whisk until combined; the batter goes from gloopy to very liquid and slack.

Spray two eight-inch rounds, line them with parchment rounds, and divide the batter between them, weighing it if you want them even.

Wrap damp cake strips round the pans so the edges do not set early and dome the cake.

Bake at 350 for 28 to 30 minutes, until a skewer in the centre comes out clean.

For the filling, whisk the granulated sugar and all-purpose flour together in a medium saucepan.

Drizzle in the milk while whisking so it does not lump.

Whisk frequently over medium heat until it simmers, tiny bubbles at the edge, about six to eight minutes.

Reduce to medium low and cook two minutes more, stirring constantly, to cook the flour through and thicken it.

Transfer to a heatproof bowl, cover the surface with paper or plastic so no skin forms, and cool on the counter about two hours to room temperature -- not ice cold.

Beat the unsalted butter two minutes on medium until creamed, scraping the bowl down.

On medium low, add the cooled milk mixture two to three tablespoons at a time, letting each addition incorporate, then the rest in batches.

Add the vanilla, raise to medium and run one to two minutes until silky and smooth.

Optionally mix in a tiny quarter drop of purple food coloring to brighten it.

Set the first layer glossy side down on a cake stand so it absorbs the cream and does not slide, spread all the filling on with an offset spatula, and add the second, completely cool, layer.

Coax the filling to the edge, smooth the seam all round, fill any gaps, and chill. It can sit covered in the fridge up to two days before the ganache goes on.

Chop the bittersweet chocolate small into a medium bowl.

Microwave the heavy cream 90 seconds to steaming, pour it over the chocolate and let it stand five minutes, then stir until silky smooth, with 10 second bursts in the microwave for any unmelted pieces.

Let the ganache thicken until spreadable, about 15 minutes depending on the room, or it will run straight off the cake.

Dollop most of the ganache on top, spread it to the edges with a small offset spatula so it drips off gracefully, smooth the sides, and clean the plate edge with a damp paper towel.

Chill until the ganache sets completely, then slice.