Making the Pastry

1. Pour the flour into a mixing bowl and add the butter, lard, and salt.  Rub the fats into the flour by pinching and mixing it with your fingers until the mixture resembles coarse breadcrumbs.
2. Gradually add just enough ice water to hold the mixture together into a dough.  Make sure that there's no flour left at the bottom of the bowl.
3. Gently knead the pastry a few times, then shape it into a disc and wrap it tightly in plastic wrap.  Chill it in the fridge for at least 1 hour before rolling.  This relaxes the gluten in the pastry and firms up the butter and lard in the dough.

Making the Butter Tart Filling

1. Preheat the oven to 200 F.
2. If you're adding raisins, pour 1/4 cup (60 ml) of boiling water on top of the raisins and let them soak while you're making the filling.
3. Whisk together the softened butter, brown sugar, corn syrup, eggs, and vanilla extract until well-combined and smooth.  Set aside.
4. Roll out the chilled pastry to 1/8" thick on a lightly floured surface.  Cut out as many 4-inch circles as you can, re-rolling the scraps to get more rounds.  Line a 12-hole muffin pan with the pastry circles, making sure to get the pastry into the corners of the pan.
5. Drain the raisins and sprinkle them in the bottom of each tart.
6. Fill each pastry shell 3/4 full with the filling.  Be careful not to overfill, as this will cause them to overflow in the oven.

Baking the Butter Tarts

1. Bake at 200 F for 15 minutes, then turn the heat up to 350 F and bake for another 10-15 minutes.  Watch the tarts closely towards the end of the baking time, as the filling can overflow.  They are done when the pastry is a pale golden brown around the edges and the filling is browned.
2. Let them cool in the pan for 5 minutes, then remove them and place on a wire rack to cool completely.
3. Serve at room temperature.  Store leftover ones in the fridge for up to 5 days, or freeze for up to 3 months.