Scald the milk and cream in a small pot and set aside to cool until warm.
Whisk together the yolks, sugar, flour and vanilla. Set on low heat and begin whisking.
Slowly pour in the warm milk mixture and continue whisking.
Whisk on low heat until mixture thickens.
Transfer to glass bowl and whisk in the melted chocolate.
Cover with plastic wrap pressing down onto the surface and allow to chill for a few hours in the fridge.
For the Swiss Meringue:
Add egg whites, sugar and salt in a bowl.
Give the mixture a brief whisk.
Place the bowl over a pan of simmering water. Make sure the water does not touch the bowl.
Whisk the egg whites occasionally while it warms up. When the mixture has warmed, whisk it constantly. You will heat the mixture until it reaches an internal temperature of 160 degrees. (I suggest using a kitchen thermometer)
Transfer the bowl to a stand mixer with a paddle attachment.
Run on low speed. Increase gradually to high until you get stiff, glossy peaks. Add vanilla, mix until incorporated. Transfer it to a piping bag or to a ziplock bag to store in the freezer.
For the Cookie Cups:
Preheat the oven to 350 degrees. Use a cupcake tin for these pastries.
Mix butter and sugar in a large bowl. Beat on medium-high for 3-5 minutes or until fluffy.
Add the egg. Again, beat on medium-high. Still in the vanilla extract and the egg yolk. Do this until the mixture is combined.
Scrape the sides of the bowl with spatula.
Add the cornstarch, baking soda, flour, and salt. Stir.
Take a small ice cream scooper and place one in each muffin tin.
Bake for about 12 minutes.
Take some kind of cylinder-like implement (e.g. a table spoon) and press into the cooked dough to create a “well.”
Place back in the oven and cool for an additional 5 minutes.
Remove pan from oven. Allow 15-20 minutes to cool. Remove the pastries from their mold.
For Assembly:
Transfer chocolate pastry cream to piping bag. Cut the tip off. Pipe into cooled cookie cups.
Fill it just below the top.
Transfer the Swiss Meringue to a cookie bag, fitted with a large, round tip. Pipe a large dollop on top (making sure to cover all the chocolate).