Start with making a basic pastry cream: whisk the sugar and egg yolks together in a bowl for 1-2 minutes until the mixture is light and fluffy. Then, add the cornstarch, flour, and vanilla, and mix until the mixture is smooth.

In a saucepan, heat the milk over medium heat until it starts to simmer (but not boil). Slowly pour the hot milk into the egg yolk mixture, whisking constantly to temper the yolks.

Return the mixture to the saucepan and cook over medium heat, stirring continuously, until it thickens—this should take about a minute after boiling. If the mixture is lumpy, keep whisking until smooth. Remove from heat and strain if necessary.

Gradually add the softened butter, mixing until fully incorporated.

Cover the pastry cream with plastic wrap and let it come to room temperature while preparing a 9x13 / 22x33cm baking dish and your Graham crackers.

Once the cream is cooled to room temperature, place very cold 36% fat-content heavy cream in a mixing bowl and use an electric hand mixer to whip the mixture for about 2-3 minutes until fluffy. Do not overwhip.

Gently loosen the pastry cream with a few quick whisks, then carefully, using a rubber spatula, fold in the whipped heavy cream, ensuring the mixture stays light and airy.

To assemble, divide the graham crackers into 3 (as there will be three layers) and the diplomat cream into 2 (as there will be two layers).

Place one-third of the graham crackers in a single, tight layer at the bottom of a 9x13-inch baking pan. Break the crackers as necessary to fit. Evenly spread half of the diplomat cream on top. Add a second layer of graham crackers on top of this. Then, spread the remaining diplomat cream. Finally, top with a third layer of graham crackers.

Refrigerate while preparing the chocolate ganache glaze.

Semi-melt the chocolate in the microwave. Meanwhile, heat the cream in a saucepan until it just begins to simmer. Pour the simmering cream over the melted chocolate and stir until smooth, then stir in the oil. Set aside and allow the glaze to cool to about 30°C (86°F). It should have a smooth consistency—neither too runny nor too thick—before spreading it all over the cake.

Pour the chocolate ganache glaze over the cake and smooth with a spatula. Chill the cake for 4 hours before serving, for the layers to soften.

Serve the Chocolate eclair cake after the 4 hours of chilling time. Any leftovers can be covered and stored in the refrigerator for 2 days. The layers will further soften over time.