Mix butter and condensed milk in a stand mixer till well combined.
Add in flour and mix at low speed till the mixture is like rice grains. Switch to spatula and mix by hand till a soft dough that’s smooth and not crumbly.
Wrap with cling wrap and rest in the fridge for 20 min.
Form 10g balls and press using a tart mould.
Bake the tart shells at 150 deg for 20 min. When time is up, take out shells and let it cool slightly.
Pipe Nutella onto tarts.
Return to bake at 130 deg C with fan for about 15 min.
Bake at 150 deg C for another 5-10 min. If you see the Nutella melting and cracking badly when you increase the temperature, then your oven is too hot, adjust temperature.
Allow to cool, then pack in airtight container. You can also chill it.
