Make dough: Blend together flour, butter, shortening, salt and sugar in a bowl with your fingertips or a pastry blender (or pulse in a food processor) until most of mixture resembles coarse meal with the rest in small (roughly pea-size) lumps. Drizzle evenly with 3/4 cup ice water and gently stir with a fork (or pulse in food processor) until incorporated.
Test dough by gently squeezing a small handful. When it has proper texture, it should hold together without crumbling apart. If necessary, add more ice water, 1 tablespoon at a time, stirring (or pulsing) until just incorporated, and test again. (If you overwork mixture or add too much water, pastry will become tough.)
Turn out dough onto a lightly floured work surface and divided into 8 portions. With heel of your hand, smear each portion once in a forward motion to help distribute fat. Gather dough together and form into 4 disks. Chill, wrapped in plastic wrap, until firm, at least 1 hour.
Roll each piece of dough with a rolling pin until 1/8th inch thick on a lightly floured surface. Cut to appropriate size and line the tart tins. Place in freezer or fridge while making filling. You get flakier pastry if you keep it chilled before baking.
Filling: Mix the brown sugar, corn syrup, butter and eggs in the mixer until smooth. Add salt and vanilla and mix well.
Place about 9 raisins in each tart shell. Add the syrup mixture til about 3/4 full. Bake in a 400 degree F. oven for about 15 to 20 minutes or until pastry is golden brown. Less time for runnier tarts, more for firm. Remove from oven and cool slightly before removing from tins.
These do freeze well (that's if they last long enough). Makes 36 or more medium tarts.
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