Dice butter. Knead flour, butter, icing sugar, 1 tablespoon elderflower syrup and egg to a smooth dough. Halve the dough, roll out on a floured work surface and cut out 12 circles (each approx. 7.5 cm Ø).
Line the greased and floured troughs of a muffin tray (12 troughs) with the dough circles. Place the muffin tray in a cool place for about 30 minutes. Wrap the other half of the dough in foil and set aside.
Line dough hollows with baking paper and add dried peas. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 25 minutes. Remove from the oven, remove baking paper with dry peas from the wells.
Allow the cake to cool slightly, lift the cake out of the troughs and let it cool on a cake rack. Clean the muffin tray, grease the troughs and dust with flour. Process the rest of the dough in the same way, bake and leave to cool.
In the meantime, halve the lemon for the filling and squeeze the juice. Boil 125 ml elderflower syrup, cream, double cream and sugar. Add lemon juice and simmer gently for about 5 minutes.
Remove from the heat and let it cool down. Spread the mixture into the tartes, let it cool down and put it in a cool place for at least 4 hours. To serve, dust with icing sugar and decorate with elderberry blossoms.