Pre-heat your oven to 180℃ (fan forced) and grease one large or four small springform tart tins.
In a food processor, combine the butter and buckwheat flour until a crumb like texture forms. Thermomix 20 seconds, speed 4.
Add in the remaining ingredients and mix until the dough forms a disc. Thermomix 30-40 seconds, speed 4. As you mix it it should come together.
Wrap the dough in baking paper and pop in the fridge for 1 hour.
Cut the dough in half (you have enough for two large tarts or 8 small tarts - see tip below). So for this recipe, you only need half of the dough, the other half can be refrigerated or frozen until needed.
Roll the dough to about 5mm thickness. It's important to work quickly, especially in a hot kitchen - as the dough softens, it's harder to work with. Cut the pastry to suit the tin you are using. You may need to bring the pastry back together and re-roll to use it all up. You can mend any tears or thin areas by pushing the dough into the tin with your fingers.
Any leftover you can cut into shapes to decorate or just enjoy as biscuits.
Cover the tarts with a sheet of baking paper and weight it to blind bake the shells.
Cook for just 7 minutes for a small tart, 10 for a larger one. Remove from the oven and cool in the tins for a few minutes, then, when cool enough to handle, carefully push out of the base and cool completely on a wire wrack.
Keep in an airtight container for 1-2 days until ready to serve.