Soak raisins in warm water.
Pour the flour and sugar in the bowl of a stand mixer. Dissolve the yeast in a cup with half the water, then pour it in the bowl and knead on low speed with the dough hook, adding gradually the remaining water.
When the dough has completely absorbed the water, add the egg yolk and the butter cut into small pieces. Knead on low speed until the dough becomes smooth, but still slightly sticky.
Add eventually the aniseed, the squeezed raisins and salt and knead just enough to get them incorporated in the dough.
With oiled hands, remove the dough from the bowl and collect it on a well greased wooden board. Form a ball and place it in a bowl previously drizzled with olive oil.
Cover the dough with a damp cloth and let it proof for about three hours in a warm place.
After this time, slide back the dough onto the oiled wooden board, knead it gently and form a long thin sausage.
Line a baking sheet with parchment paper, place a cup in the centre and wrap the dough around it, then seal together the ends to form a donut.
Cut the dough all the way around, so that it will rise better, and let rest for at least an hour.
Preheat the oven to 180°C (350°F - gas mark 4). Melt the sugar into the hot water to make a syrup, then pour it over the egg white and beat with a fork until frothy.
Brush the buccellato with the syrup of water, sugar and egg white and bake for about 45 minutes, until golden brown. Let it cool completely before slicing.