Preparation time: 45 min. + cooling time
1. Using the whisk attachment on an electric hand mixer, whisk the egg yolks with the butter and sugar until light and fluffy. Add the flour and lemon zest and mix at a low speed. Wrap the dough in cling film and chill for 1 hour.
2. Pre-heat the oven to 160 °C (top/bottom heat).
3. Form the dough into a roll on a slightly floured work surface and then divide into 32 similar-sized pieces. Form each piece of dough into balls with floured hands and place them onto a baking tray lined with baking paper. Make a small dent in the centre of the dough ball using your finger or the handle of a wooden spoon. Press some decorating sugar on the edge of the dough around the dents as desired.
4. Place the tray in the hot oven on the second shelf from below for 8 minutes. Heat the jam in a small pot and stir until smooth. Take the tray out of the oven, carefully fill the dents of the jam thumbprints with jam using a small spoon and place the tray in the oven for another 4 minutes.
5. Remove the tray from the oven, leave the jam thumbprints to cool on a cooling rack and then store in airtight containers.