For this recipe you will need:
Make sure that all the ingredients – even the egg – are cold. This dough dislikes warmth, and will get sticky, so cool your hands by running them under cold water before you knead it.
2) If you have a food processor, add all your cold ingredients, and pulse until the dough just comes together. Don't overwork the dough! That develops gluten and your dough will get hard and lose its airy crunch. Move to step 5 from here
If you make the dough by hand:
1) Sift flour in a pile, and with a spoon create a well in the middle.
2) Add sugar to the well, and the egg.
3) Cut the cold butter into pieces and place all around on the edge of the flour well.
4) Thoroughly chop all the ingredients with a knife and knead as quickly as you can to prevent warming. Knead pieces together into a ball. Make sure not to overwork the dough.
5) Wrap the dough in plastic and let rest for 30 mins up to 24 hours in the fridge
6) Preheat your oven to 400 degrees Fahrenheit
7) To assemble the thumbprints, use a one tablespoon cookie scoop or measuring spoon to measure out the dough.
8) Next, roll the cookie dough into balls and toss each one in a little granulated sugar (the sugar is optional, it provides a bit of crunch).
9) Make an indentation in each cookie. You can use your thumb, but I like to use the handle of a wooden apple corer, or you could use the back of a wooden spoon. Dip your utensil into a bit of flour so the dough won’t stick.
10) Fill each indentation with 1/2 teaspoon of your favorite flavor of K’s European Jam. The jam will shrink a bit in the oven, but be careful not to overfill so it won't run over.
11) Bake for about 15 minutes. They should be baked through but still pale on top.