Activate the Yeast: In a small bowl, combine warm milk and yeast. Let sit for 5-10 minutes until frothy.
1 package active dry yeast, 3/4 cup milk
Make the Dough: In a large mixing bowl, combine flour, sugar, lemon zest, and salt. Add the yeast mixture, eggs, and softened butter. Knead until a smooth, elastic dough forms (about 8-10 minutes by hand or 5 minutes in a stand mixer).
3 1/2 cups all-purpose flour, 1/3 cup granulated sugar, 3 large eggs, 1 tsp lemon zest, 1/2 tsp salt, 6 tbsp unsalted butter
First Rise: Place dough in a lightly greased bowl, cover with a damp cloth, and let rise in a warm place for about 1 hour or until doubled in size.
Shape the Donuts: Punch down the dough and roll it out to about 1/2 inch (1.3 cm) thickness. Use a round cutter to cut out circles. Place them on a baking sheet lined with parchment. Cover and let rise another 30 minutes.
Fry the Berliners: Heat oil in a deep pot to 350°F (175°C). Fry the donuts in batches, about 1-2 minutes per side, until golden brown. Drain on paper towels.
Vegetable oil
Fill and Sugar: Once cooled slightly, use a piping bag to inject jam into each donut. Roll in sugar to coat.
1 cup jam, 1/2 cup granulated sugar
Serve: Enjoy fresh for the best texture and flavor.