Start with the pastry cream, in a medium bowl, whisk together the egg yolks, sugar, flour, cocoa powder, salt and cornstarch until smooth.
In a small saucepan over low heat, add the milk and warm until it starts to steam. Once it is steaming, add the melted chocolate and whisk to incorporate. While whisking the egg mixture, slowly pour in about one-third of the milk. Once it is combined, continue to add the rest of the milk, whisking continuously. Return the mixture to the saucepan.
Place the saucepan over medium heat. Cook until thickened, whisking constantly. Stop to check if the cream starts to bubble. If it does, cook for another minute. Remove from heat and stir in the vanilla extract. Transfer the pastry cream into a medium bowl. Cover with a piece of plastic wrap that is pressed right up against the surface of the cream. Transfer to the fridge and cool completely.
For the tart dough, in a large mixing bowl with an electric mixer fitted with a paddle attachment, beat together the butter, sugar, salt and vanilla on medium speed. Mix until smooth. Scrape down the bowl if needed. Add the yolk and mix until blended. Pour in the flour. Mix on low until the dough is crumbly, about 1 minute.
On a lightly floured working surface, press the dough together into a round disk. The dough may be a little crack and dry, it is ok. Wrap the disk with plastic wrap and chill in the fridge for 30 minutes.
Preheat the oven to 400˚F (200˚C).
Take out the chilled dough. Cut into 8 equal portions. Roll each portion into a round ball. Flatten each ball into flat disks with hands. Place a flat disk into a tart tin and mold the dough into a tart shape. Make sure there is no crack. Poke holes at the bottom of the tart with a fork a few times. Place the tart shells on a baking sheet. Bake in the oven until golden brown throughout, about 12 – 16 minutes. Remove from oven and cool for 10 minutes. Tap the tart tin bottom lightly on the table a couple times. Turn the tart tin over and remove the tart shells carefully. Let them cool completely on a cooling rack.
When the tart shells are cool. Brush the inside of the tarts with melted chocolate, but leaving the top edge clear. Transfer the shells to the fridge to harden, about 5 minutes.
To assembly, stir to loosen up the cooled pastry cream. Scoop and fill each tarts. Dried the strawberries thoroughly with paper towels and arrange 3 – 4 strawberries on top of each tart.
In a small microwave-safe bowl, combine the apricot jelly and water. Microwave for 15 – 20 seconds or until warm and the jelly is runny. Brush and cover the strawberries with warm jelly. Serve immediately or transfer the finished tarts to an airtight container and keep in the fridge. Enjoy within 4 hours.