Place the sugar in the Thermomix Bowl. Mill for 10 seconds / speed 10.
Chop the butter into pieces and add to the bowl with the flour, egg, salt and vanilla extract. Mix for 30 seconds / speed 5.
Turn out onto a floured surface and quickly bring it together with your hands. Shape into a flattened disc.
Wrap the pastry in plastic wrap or place in an airtight plastic bag and chill in the fridge for at least 30 minutes.
Once ready, pre-heat the oven to 180C / 350F. Place a baking sheet into the oven. Prepare your tart tin by greasing and flouring.
On a floured surface, roll out your pastry to slightly larger than the size of your tart tin. Remember to allow enough to go up the sides of the tin and for some overhang.
Place the pastry into the tart tin, being careful to press it firmly into the tin base and sides. This will help prevent air bubbles when it bakes. You can let the pastry overlap the edges slightly - you can break it off easily once it is baked and it will leave a neat edge .
Now take a fork and prick the pastry base all over - again this will help to prevent air bubbles.
Put the tart tin onto the pre-heated baking sheet and bake for 15 minutes. Set aside to cool. Meanwhile make the chocolate filling.
Reduce the oven temperature to 160 C / 320 F.