In a large measuring cup, whisk together milk, cream, and egg yolks until egg yolks are broken up and mixture is thoroughly combined.

In a medium-sized saucepan, whisk together sugar, cornstarch, and salt.

While whisking continuously, slowly drizzle egg mixture into the saucepan. Go slowly so you can incorporate all ingredients and eliminate any lumps that might otherwise form.

Turn stovetop heat to medium and continue whisking constantly until mixture thickens to a pudding-like consistency. This may take several minutes, don’t turn up the heat to speed up the process. You can tell it’s done when the whisk leaves trails through the mixture and it coats the back of a spoon.

Remove from heat and pour immediately through a fine mesh strainer into a large, heatproof bowl.

Immediately add chopped chocolate and butter and stir until melted.

Stir in vanilla extract, mixture should be completely combined and smooth.

Allow pastry cream to cool at room temperature for 15-20 minutes, stirring occasionally, then place a piece of plastic wrap against the surface of the pastry cream to prevent skin from forming. Transfer to the refrigerator and allow to chill while you prepare your craquelin.

In a medium-sized mixing bowl, beat together butter and sugar until well combined.

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

Stir dry mixture into butter mixture until completely combined and you have a crumbly mixture that clings together.

Use your hands to form dough into a ball and flatten into a disk. Place dough between two pieces of parchment paper and use a rolling pin to roll out to a ⅛” thickness (this will roll out to roughly 11x9”).

Place on a baking sheet, remove the top piece of parchment paper, and use a 2” round cutter to cut circles into the dough, leaving the circles on the tray. ( If it is too difficult to cut, you can place it in the freezer for several minutes and then try again). Transfer the baking sheet to the freezer and freeze until firm, about 30 minutes.

Preheat oven to 400F (200C) and line a large baking sheet with parchment paper.

In a medium sized heavy-bottomed saucepan, combine water, butter, and salt. Heat over medium/high heat and bring to a rolling boil.

Reduce heat to medium/low and add flour, stirring continuously with a spatula until the mixture forms a ball that pulls away from the sides of the pot.

Remove from heat and continue stirring for another 1-2 minutes to allow steam to release and the mixture to cool slightly.

Add eggs one at a time, stirring until each egg is totally incorporated before adding the next. The mixture may look like it's separating or broken at first, this is normal, just keep mixing until it becomes smooth and velvety.

Transfer mixture to a piping bag with the end cut off, or fitted with a 1A tip, and pipe onto your prepared baking sheet, making mounds that are 1.5” (4cm) wide by 1” (2.5cm) tall, and spacing at least 1.5” (4cm) apart from each other. When piping you want to pipe straight down, not swirling like you would when applying cupcake icing.

Remove your craquelin from the freezer and place one cut out circle of craquelin on top of each pastry mound, very gently setting on top.

Bake in preheated oven for 35-40 minutes, or until pastry appears dry and a light golden brown in color.

Remove from oven and quickly (and carefully!) use a sharp knife to pierce each puff lower on the side, near the bottom, to release steam. Pierce each halfway through the pastry, then return to the baking sheet. Turn off the oven and return the baking sheet to the oven with the door cracked slightly open for 10 minutes.

After 10 minutes, remove from oven and allow to cool completely on baking sheet before proceeding.

Transfer chocolate pastry cream into a large piping bag (I fit mine with a Wilton 9 tip). Once choux pastry has cooled completely, fill each with pastry cream by inserting the piping bag into the hole you pierced with a knife, and slowly fill, dividing pastry cream evenly between all pastries.