For the chocolate cake:
Preheat the oven to 350 degrees F.
Grease 2 9-inch round cake pans and line the bases with parchment paper. Set the prepared pans aside.
Combine the sugar, flour, cocoa powder, baking powder, baking soda, and salt in a large bowl. Add the eggs, buttermilk, oil, and vanilla and whisk together. Add the boiling water to the bowl and whisk to incorporate well. Divide the batter equally between the 2 prepared pans.
Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool in the pans for 10 minutes, and then turn the cake out onto a wire rack. Remove the parchment from both layers. Turn one of the cakes right side up to be the top layer. Allow the cakes to cool completely, about 1 hour. Once cool, place the first cake layer, with the bottom of the cake facing up, on a cooling rack set over a baking pan or on a cake plate and spread the jam on top of the cake, leaving a 1/2-inch border of cake. Top the jam with the second cake layer, that has been turned right side up. Pour the ganache over the cake, if using.
For trifle toppings:
Put the strawberries in a small bowl and sprinkle with sugar to the desired sweetness. Set the bowl aside. In another bowl, add the cream and softly whip with a hand whisk or electric mixer, and set aside until needed.
Cut one slice of cake and lay in a dessert dish. Sprinkle the cake with about 1 tablespoon of sherry. Spoon 1/3 cup of the sugar and strawberries mixture over the cake, and then top with a large spoonful of whipped cream. Add a fresh mint sprig for garnish.
To make chocolate ganache:
In a small saucepan, heat the heavy cream over medium heat until little bubbles appear around the edges. Remove the saucepan from the heat and immediately add the chocolate. Shake the pan to submerge the chocolate and leave it to melt for a couple of minutes. Stir gently to mix the chocolate to a smooth cream. Set the mixture aside for 20 to 30 minutes to thicken to a spreadable consistency. Stir through a few times as it's cooking over low heat to keep the ganache smooth.
Pour the thickened ganache in a wide ribbon around the circumference of the cake and with a spatula gently push the ganache over the edges. It should fall in thick waves. Fill in the middle with more ganache and swirl the ganache into luscious billows.