Jelly Doughnuts (Sufganiyot)

This recipe is from For the Love of the South: Recipes and Stories from My Southern Kitchen.

In a stand mixer fitted with the paddle attachment, combine the flour, sugar, salt, yeast, eggs, lemon zest, and water. Mix on medium speed for about 8 minutes or until the dough pulls away from the sides and forms a ball.
Allow the dough to rest for one minute, then continue mixing and slowly add the butter. Once the butter is incorporated, turn to high speed and mix until glossy and smooth, about another 5 minutes.
Put the dough into a lightly oiled bowl, cover loosely with plastic and let rest until doubled in size, about 1 to 1 1/2 hours. Knead the dough in the bowl, then cover and refrigerate overnight or up to 24 hours.
Line a baking sheet with parchment paper and dust with flour. Divide the dough into 12 equal pieces, shape them into balls and then place them on the prepared baking sheet. Cover with plastic and let rest until doubled in size, about 2 hours.

In a heavy-bottomed medium pot, heat about 3 inches of oil over medium heat until it reaches 360°F on a thermometer. Line a plate with paper towel. Once the oil it hot, carefully slide the dough into the oil. Only put a couple in at a time, so they have plenty of room to float. Allow to fry for about 2 minutes, then flip them over to cook the other side. Drain the doughnuts on the paper towel for a minute, then dredge the doughnut in the vanilla sugar. Repeat with the rest of the dough.
Once the doughnuts have cooled slightly, pierce the "seam" of the doughnut with a paring knife, creating a pocket for the filling. Use a pastry bag with a round tip (make sure it is big enough for any fruit to fit through) or use a gallon-sized plastic bag as Amber does. Fill each doughnut.
Top the jelly doughnuts with a fresh blackberry and serve immediately.