How to make all-butter pastry shells for butter tarts
In a large bowl, whisk together the flour, salt, and sugar.
Add the cubed cold butter to the bowl and toss it in the flour to coat it.
Working quickly with your palms and fingertips, rub handfuls of flour and butter together to work the butter into the flour until you get a mixture that has a coarse, sandy texture, with pieces of butter no larger than a chickpea. The mixture will have a golden yellow colour when you are done.
Sprinkle the mixture with the water. Work the liquid ingredients into the sandy mixture, whisking it in with the fork.
Clean the fork with your hands and switch to working the dough with your hands, quickly kneading it all together until you get a smooth dough.
Split the dough into two; roll each into a log, wrap in plastic wrap and chill for at least 1 hour.
When you are ready to roll the dough, simply divide each log into 6 pieces, rolling each piece into a 4 inch circle roughly, using a rolling pini. Carefully work each disk into the well of a 12 cup muffin pan.
Chill the tart shells in the fridge for at least 15 minutes while you preheat the oven to 400 ºF.
When the tart shells are very cold and firm and the oven is preheated, line each tart shell with a silicone muffin cup and fill with dried beans or pie weights.
Place the muffin pans on the bottom rack of the oven and bake them for 20 minutes, the edges will begin to dry out and turn golden when they have baked sufficiently.
Take them out of the oven when they are blind-baked, and let them cool slightly so that you can remove the silicone liners and pie weights.
How to make butter tart filling
Drop the oven to 375 ºF (190 °C).
In a medium bowl, stir together the melted butter, the brown sugar, the cream, the vanilla, the salt, and the egg,. Stir well.
If you want to make tarts with raisins or nuts, scatter a few at the bottom of each crust (see notes).
Divide the filling among the blind-baked tart shells. You'll place roughly 30 grams of filling per tart.
Bake the tarts on the middle rack of the oven for another 15 minutes. The edges of the crust should be golden and the filling will be puffed and golden, but will not be boiling yet. If it's boiling, the filling will be overcooked and sink.
Remove the muffin pans from the oven. Let the tarts cool for about 5 minute until they are firm, then gently twist them to make sure they will unmould when cooled.
Once cooled, unmold the tarts with the help of an offset spatula.
Notes
Ask a Canadian about butter tarts and whether they should contain raisins, dried currants, or nuts, and you will probably get a very passionate response. I prefer them without, but feel free to add up to ½ cup total of raisins or dried currants or toasted nuts, or a combination of these (adding up to ½ cup).