To make the cake: While the custard is chilling, prepare the cake:
Preheat oven to 350°.
Beat together the sugar, butter, salt and vanilla extract in a stand mixer or in a large bowl with a hand mixer until the mixture is fluffy. Beat in the oil, scrape down the sides, and then add the eggs, one at a time, beating until the mixture is even fluffier.
In a separate bowl, whisk together the flour, cornstarch and baking powder.
Alternate adding about 1/3 of the dry mixture and 1/3 of the milk, scraping down the sides of the bowl after each addition, until the batter is even and smooth.
Grease and flour an 8-inch-by-8-inch square baking pan or a 9-inch round baking pan and spoon the batter into the pan. Bake for 38-45 minutes, or until the cake tests clean with a skewer or tooth pick and the edges pull away from the pan. Let the cake cool in the pan for 10 minutes before turning onto a rack to cool completely.
To make the ganache: While the cake is chilling, prepare the chocolate ganache:
Pour the heavy cream into a heavy-bottomed saucepan over medium high heat and bring it to a boil. Pour over the chopped chocolate and let rest for 5 minutes, undisturbed. When the 5 minutes have elapsed, stir slowly in one direction until the mixture becomes smooth and glossy. Add the pinch of salt and stir in gently. Set aside for 10 minutes at room temperature.
To finish: Fold the whipped cream into the chilled pastry custard and set aside.
Divide the cooled cake into 4 equal portions. Crumble 1 portion of the cake over the bottom of the trifle dish, top with 1„3 of the pastry cream and drizzle with about ¼ cup of the ganache. Repeat the layers, ending with a top layer of cake and all of the remaining ganache. Let stand for 30 minutes before serving.
-- Adapted with thanks from King Arthur Flour's Boston Cream Pie recipe.
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