Preheat the oven to 175 degrees Celsius/ 350 degrees Fahrenheit. Line two baking trays with parchment paper.
Dough: Place the flour, soft butter, sugar, and egg into the stand mixer bowl and process shortly until the mixture resembles streusel. Alternatively, mix the dough in a regular bowl with a hand mixer.
Form cookies: Turn the dough onto the working surface and knead very shortly to bring the dough together and form a ball. Break small pieces of dough and form little balls about as large as a cherry. Press with the thumb in the middle of each ball to create a well.
Fill with jelly: Mix the jelly or jam very well with a spoon to make it smooth and easily spreadable. Next, add a small amount of jelly or jam into each well using a teaspoon.
Bake: Place the first batch of thumbprint cookies on one of the prepared baking trays. Next, place in the preheated oven and bake for about 10 to 15 minutes until very slightly golden. Check the first batch repeatedly after the first 10 minutes, the cookies should not get too dark, they should remain pretty pale.
Second batch: While the first batch of cookies is in the oven, prepare the second batch. Place the new batch on the second prepared tray (never place the cookies on a hot baking tray, they will spread too much).
Rest: Leave the baked cookies on the baking tray for about 10 minutes, then transfer them to a wire rack to cool completely.
When cool dust with icing sugar, if you wish.
Store: They will keep for at least one week in an airtight container, arranged between sheets of waxed or parchment paper.