In a large bowl, combine egg yolks, sugar, sugar, cornstarch, and salt. Use a whisk and mix until combined and the sugar is dissolved.
Add the milk to a saucepan and heat medium-low heat. Stir until it simmers for about 5 minutes. Don't let it boil, keep stirring to avoid overflowing.
Slowly add the milk to the egg yolk mixture about 1/4 cup at a time. Whisk constantly to prevent curdling.
Return the mixture to the saucepan and cook over medium heat, whisking constantly to prevent curdling. Add butter and vanilla extract, and stir to combine.
Once the custard has thickened, strain it through a fine-mesh sieve to remove any lumps or bits of cooked egg. See NOTE #1
Pour the custard into a bowl and cover with plastic wrap, pressing the plastic wrap onto the surface of the custard to prevent skin from forming.
Chill the custard in the refrigerator for at least 2 hours before using it as a filling. Remove from the fridge and stir to help it flow more easily into the decorating bag.
Spoon the custard into the decorating bag or ziplock bag, and push it down into the bag. Cut the tip of the bag with kitchen shears. Gently squeeze the bag to release the custard into the tart shell, filling it about 2/3 of the way full.
Use a spatula and smooth the top of the tart. Repeat the process with the remaining tart shells, refiling the decorating bag as necessary. See NOTE #2 Heat the microwave apricot jam in a microwave for a minute. Meantime top the tart with raspberries.
Use a pastry brush and brush the fruits with apricot jam to give them a shiny look. Garnish with mint leaves. Refrigerate for an hour before serving.