Pastry Cream: Combine milk, cream, ⅓ cup sugar, and salt in a medium-sized saucepan over medium heat. Stir frequently until sugar is dissolved and mixture begins to simmer. Remove from heat and allow to cool for 10 minutes.

In a separate large bowl, vigorously whisk together egg yolks and remaining sugar until sugar begins to dissolve (about 15-30 seconds). Sprinkle cornstarch over mixture and whisk until combined and slightly thickened.

While whisking constantly, slowly drizzle about ⅓ cup of milk mixture into the egg mixture (we’re tempering the mixture this way to keep the eggs from cooking). Gradually add remaining cream until completely combined.

Pour mixture back into saucepan and return to the stovetop over medium heat, whisking frequently until mixture begins to thicken. Remove from heat and pour through a fine mesh strainer into a heatproof bowl. Add vanilla and whisk in butter, one piece at a time, until completely melted and mixture is smooth.

Place a piece of plastic wrap directly against the surface of the pastry cream, allow to cool at room temperature for 20 minutes then transfer to refrigerator to chill completely (about 2 hours). Meanwhile, prepare cupcakes.

Cupcakes: Preheat oven to 350F (175C) and line two 12-count cupcake tins with paper liners. Set aside.

With an electric or stand mixer, beat together butter, oil, and sugar until light, fluffy, and well-combined (about 30 seconds on medium/high speed).

Add eggs, one at a time, stirring well after each addition, and stir in vanilla extract.

In a separate bowl whisk together flour, baking powder, and salt.

Using a spatula now rather than the electric mixer, gradually alternate adding the flour mixture and buttermilk to the butter/oil batter, starting and ending with the flour mixture and stirring until just-combined after each addition. Ingredients should be thoroughly combined, but take care not to over-mix or your cupcakes will be dense and dry.

Fill cupcake liners no more than ¾ full (if you have extra batter discard it, don’t overfill the liners or your cupcakes will cave in), transfer to center rack of oven (I usually bake one pan at a time to ensure even baking), and bake for 17-19 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Allow to cool for 10 minutes in cupcake pan then transfer to wire rack to cool completely.

Frosting: Shortly before you’re ready to assemble, prepare chocolate frosting by combining chocolate chips, heavy cream, and corn syrup in a large heatproof bowl. Heat in the microwave in 25 second increments, stirring well in between, until chocolate is completely melted and mixture is smooth. Sift in powdered sugar and allow to cool for about 10 minutes (you can carve and fill your cupcakes during this time) or until ganache is still fluid but not so runny that it will run right off the cupcakes.

Assembly: Once cupcakes are cooled and pastry cream is chilled, fill a large piping bag fitted with a round tip (or use a ziploc bag with a corner cut off) with pastry cream. Use a knife to cut a 1 ½ ” wide and 1” deep cavity into each cupcake (don’t cut all the way to the bottom!). Remove this core from your cupcake and then slice off the bottom so only a ¼” thick piece of the surface remains (see pictures in post or video for a visual).

Fill cupcake with pastry cream and top with reserved ¼” thick cupcake surface, fitting it snugly against the pastry cream. Drizzle ganache generously over the top of each cupcake, then allow ganache to harden before serving.

Store cupcakes in the refrigerator in an airtight container for up to 5 days.