In a food processor, blend 300g flour, 2 tablespoons sugar and salt. Add cold (almost frozen) butter, and process in short bursts until the the mixture is crumbly. Should take only a few bursts to make the pieces of butter a little smaller than pea-sized. Add in chilled water, about 140ml, and quickly process. Dough should still look crumbly. Transfer pastry mixture to a mixing bowl, and set aside.
Using the same food processor bowl, chop pecans. Transfer chopped pecans to a small bowl, and set aside.
Again using the same food processor bowl, add melted butter and brown sugar; pulse a few times to mix. Add eggs and vanilla, and pulse until blended. Mix in pecans with a few more pulses. Don't try to make light and fluffy; just use a few pulses to blend everything together.
Add one dessertspoon of pastry crumbs into each cup of a mini muffin/bun tin. Lightly press down dough to make a small dimple. You are not trying to mould to fill cups completely.
Spoon one dessertspoon of pecan filling into each cup.
Bake in preheated oven for 25 minutes, or until lightly browned. Let cool in tins for a minute or two, then use the end of a table knife to turn each tart out of its tin.