Add the icing sugar, butter, vanilla & salt in the bowl of a stand mixer. You can also use a hand mixer.
Beat the mixture on a medium speed for 2 minutes until it forms a paste. Scrape down the bowl after 1 minute.
Add the egg yolks and mix until they're fully incorporated.
Slowly add in the flour, and mix on low speed until it is just combined. The mixture should look crumbly but should not be a giant ball of dough.
Pour the mixture on a lightly floured surface. Split the dough into three batches.
Place a large piece of parchment paper down, lightly flour it, and place one batch of dough on top. Sprinkle the dough with flour and place another large piece of parchment paper over the top.
Roll the dough out with a rolling pin to about 3mm and roughly into the shape of a circle. (be careful it’s not too thick).
Check the thickness by lifting up the side of the parchment paper. Place the whole sheet of pastry in the fridge for 30 minutes. Repeat these steps with the second batch of dough.
Once the dough has chilled, remove one sheet from the fridge. Peel off the top sheet of parchment paper, very lightly flour the pastry, then flip it over.
Peel off the remaining sheet of parchment paper, lightly flour the dough again to prevent it from sticking. Lift the pastry onto a baking tray lined with a silicon mat.
Using a 4.5" round perforated tart ring (or any tart ring you have) cut out 4 large circles of dough. Peel off the excess dough and you can re-roll this for another use.
Remove the second sheet of dough from fridge. Using a sharp knife, cut long strips of dough about 7-8” long and ¾” wide.
Carefully lift the strips and place them on the inside of the tart rings. Press them against the side so they line up with the dough at the bottom.
Press the bottom of the dough gently with your fingers to mold it to the base of the dough.
Cut off any excess pastry hanging over the top of the tart rings with a sharp knife.
Place the tart shells into the freezer for 30 minutes to 1 hour.
Pre-heat your oven to 320F. Place the tray with your tart shells in the oven. Bake for 16 minutes or until the shell is lightly golden.
Remove tart shells from the oven and allow time to cool before piping so the filling does not melt.
For the Filling
In a stand mixer, cream the butter for about 5 minutes using a paddle attachment. Add the salt. Slowly beat in the confectioner’s sugar.
Add the Nutella and increase speed to medium. Mix until incorporated.
Add the cream. You may add the optional vanilla at this step. Mix until you have a fluffy, even consistency.
For the Ganache Center
Pour powdered gelatin into a small saucepan with the cold water. (It's important to use platinum gelatin and measure the water precisely). Allow it to sit in the pan for 10 minutes.
Place cream into a medium saucepan with vanilla paste & coffee.
Place the saucepan of gelatin over low heat to melt the gelatin, not boil it. Remove from heat once melted.
Once the cream is hot and steaming (not boiling), pour the melted gelatin in. Whisk together quickly, then immediately pour the mixture over the finely chopped chocolate. Allow it to sit for 3 minutes then whisk until smooth.
Cover cream with plastic wrap and chill in the fridge for 4 hours.
Chocolate Custard Top
Finely chop the chocolate and put in a medium bowl with a sieve placed on top. Set it aside.
Place eggs and sugar into a bowl and whisk for about 2 minutes, until the mixture is light and slightly thicker.
Pour milk and heavy cream into a saucepan over medium heat. Once it starts to steam, slowly pour it over the eggs and sugar whisking constantly.
Pour the entire mixture back into the saucepan on a medium heat and whisk it constantly until the mixture reaches an internal temperature of 167F. If you don't have a thermometer, whisk for about 45 seconds.
Pour the mixture over the sieve, onto the chocolate. Whisk together until smooth and the chocolate has melted.
Carefully pour the mixture into 4 x 4.5" Silicon Disc molds and freeze for 3 hours minimum.
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. Making 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.
Heat the mixture until it reaches an internal temperature of 160ºF or until it's not grainy between your fingers.
Transfer to the bowl of a stand mixer with a whisk attachment. Whisk until you get room temperature glossy peaks.
Sugar Spiral
Heat 1 cup or 200g of sugar up over medium heat until golden. Wait until the sugar thickens up but is cool enough to touch.
I used a long paint brush to twist the sugar around but you can use any cylindrical tool that allows you to slide the spiral right off.
Using a fork, wrap the sugar around the tool to create a spiral. Wait a moment then gently remove the spiral.
Place your sugar spiral or whatever garnish you may choose; gently on top of the tarts and enjoy!
Notes
TART SHELL NOTE
If you are using perforated rings you shouldn't need any baking paper or baking beans, but if not, it might be advisable to use baking beans to blind bake the tart.
During the first few minutes of baking keep a close eye on the pastry - if any of the pastry starts to slip down the tart ring or fall out of place, just pull the pastry out of the oven and carefully press it back into place with your finger and it should stay there.
CHOCOLATE CUSTARD TOP NOTE
If you don't have one, you can improvise by lining a tart ring with acetate and freezing it in that, or freeze the mixture in a large tray then cutting out circles once it is frozen - you might just need slightly more mixture if you do it this way though.