PRO TIP: Make the custard filling FIRST. (You can even make it ahead a day or two in advance and store in the refrigerator, in an air-tight container.)
Whisk together the egg yolks and sugar until completely combined.
Add flour, whisk very well and set aside while heating up the milk.
Heat milk, heavy cream, butter and salt in a sauce pot, over medium heat, until starting to simmer and turn off the heat.
Slowly add the heated milk mixture, 1/2 cup at a time, into the egg yolk mixture while stirring constantly. Whisk well but slowly.
Return the mixture into the pot, turn the heat back on medium to medium-low, and bring to simmer while stirring constantly. (Do not boil.) The mixture will thicken within about a minute or two. Take off heat when it thickens, but do wait to it to thicken!
Whisk in the vanilla extract and set aside to cool to room temperature.
Cupcakes:
Preheat the oven to 350℉ and line a muffin pan with cupcake liners.
Sift flour, baking soda, salt and baking powder together and set aside. Make sure your other ingredients are ready.
In a bowl of an electric mixer (use paddle attachment), beat egg whites for a few seconds and slowly start pouring in sugar. Beat for 3-4 minutes, until white and fluffy.
Start pouring in oil, slowly, as the eggs are still beating on medium-high.
Add vanilla and lower the speed to low.
Alternating, add flour mixture and buttermilk in two parts. Mix until just combined. Make sure to scrape sides and bottom of the bowl and mix until all incorporated.
Divide batter among the cupcake cups, filling them about 2/3 of the day.
Bake for 18-20 minutes. Let cupcakes cool for just a few minutes before filling, not all the way. It's better to fill when while still warm.
Filling the Cupcakes:
Use a long, thin decorating tip and fit it onto a piping bag. Fill a decorating bag with the cooled custard filling. Insert the tip into the center of the baked cupcakes while they are still pretty warm. Squeeze out about a tablespoon or so of the custard.PRO TIP: watch out for the cupcakes cracking. If they are cracking, they're starting to overfill.
Ganache Frosting:
Chop the chocolate bar into small pieces. The smaller the better it will melt.
Bring the heavy cream to simmer, be careful not to let it actually boil!
Pour the boiling cream over the chopped chocolate and stir until melted. Cool for a few minutes, to a little warmer then room temperature.(If you notice your chocolate is not melting as well, pop it into the microwave for 15 seconds on half the power. Whisk until smooth.)
When the cupcakes are completely cooled, spoon the ganache frosting on top of the cupcake and let is slowly spread. If it does not spread too easy, help it with the edge of the rubber spatula or a spoon carefully.
Notes
*Note: this custard recipe makes extra custard. Some people like to fill their cupcakes more than others, so it's better to have some leftover than not enough. If you have leftover custard, simply enjoy is as a pudding.