Grease and line the bottom of two 8” cake pans with parchment paper, grease the top of the parchment paper and the side of the cake pans. Sprinkle flour on the side of the cake pan and tap out the excess. Set aside.
In a small mixing bowl, sift together flour, sugar, salt, baking powder and baking soda. Stir with a whisk to distribute evenly. Set aside.
Add buttermilk, vegetable oil, egg, and vanilla extract to a medium bowl. Beat together with a whisk until smooth. Add the flour mixture to the wet ingredient mixture and fold until just combined.
Divide the cake batter among the two prepared cake pans. Bake in the middle oven rack for 27 - 28 minutes until a toothpick inserted in the middle of the cake comes out clean with just a few crumbs attached.
Remove from the oven and allow to cool to the touch, remove cakes from the pans and allow to cool completely on a wire rack.
Make the pastry cream
Heat half and half with ¼ cup of sugar in a medium saucepan over medium heat, stirring occasionally, until simmering. Take off the heat.
Beat egg yolks with the remaining sugar and salt in a medium bowl until pale in color and creamy. Add the cornstarch and whisk until smooth.
Temper the hot half and half into the egg mixture by adding a few tablespoons of hot liquid at a time while whisking constantly. Once finished, pour it all back into the saucepan.
Put the saucepan back on the heat, and cook the mixture on medium low heat, whisking constantly, until it thickens and bubbles burst at the surface, about 4-5 minutes.
Remove from the heat and stir in the butter and vanilla until the butter is melted and well mixed.
Run the pastry cream through a fine mesh sieve into a medium bowl. Place plastic wrap directly on the surface and allow it to cool to room temperature. Refrigerate until ready to use.
Make the chocolate ganache
Microwave the cream in a heat proof vessel for 50 - 55 seconds, watching it during the last few seconds to ensure it doesn’t boil over.
Add the chocolate in a small mixing bowl and pour the hot cream over it. Allow it to stand for a minute or two before whisking it until the chocolate melts completely and the mixture is smooth. Let the ganache cool completely and thicken slightly for about an hour.
Assemble the cake
If the cakes are slightly dome, level it off using a serrated bread knife.
Place the first cake layer on a cake stand or plate, add the pastry cream in the middle and level it out with an offset spatula. Invert the second cake layer and add it on top with the top of the cake sitting on the cream layer so that the flat bottom is now the top of the cake.
Drizzle the ganache over the top of the cake, allowing it to spill over the side. Serve right away or let the ganache thicken further in the fridge before serving.