Place your flour into your food processor so that it evenly covers the base of the processor bowl. Or place in a large bowl.
Grate your cold butter and sprinkle it evenly over your flour.
Pulse the food processor about 20-25 times to turn the flour and butter into a breadcrumb-like consistency. Alternatively, rub the butter into the flour by rubbing between your fingers.
Mix in your icing sugar by again pulsing a few times, or mix in with a wooden spoon if making pastry by hand.
Add your cold egg yolk and evenly sprinkle over 3 tablespoons of cold water.
Pulse again about 10 times until the mixture just starts to become a bit clumpy.
Remove your pastry dough and bring it all together into a ball, then flatten the ball so that it is about 1½ inches thick and cover with cling film (plastic wrap).
Refrigerate the dough for 30 minutes.
Grease your tartlet tins using your pastry brush, with some spare butter.
Split your chilled pastry into 18 balls, they should weigh about 28-30 grams each.
Press the dough ball out flat, so it forms a circle about 5cm in diameter. I do this in the palm of my hand.
Pop the dough circle into your tartlet case and gently press it into the base and then, moving in a circular motion and starting at the base, push the dough up into the groves, filling all of the tartlet case right to the top.
Place a teaspoon of raspberry jam (about 7.5 grams) into the base of each tartlet case, and spread out a little to cover the base.
Place in your refrigerator to chill for 15 minutes, whilst you make your tartlet filling.