For the doughnuts, heat the milk and dissolve the yeast and lemon zest in it. Melt the butter. Mix the flour, milk, butter and egg in the planet mixer using a dough hook until a pliable dough is formed. Finally, add the salt and leave to rise under a moist tea towel for 1 hour in a 40°C oven.
Shape the dough into small balls and leave them to prove in a 40°C oven for 30 minutes. Deep-fry at 170°C and store in the refrigerator.
Mix the blueberry purée with the sugar water and use this mixture to marinate the blueberries and blackberries.