• Note. Prepare the lemon curd first so it has time to cool, chill, and thicken before filling the donuts.

Lemon Curd

• Whisk ingredients. In a heatproof bowl, whisk together the eggs, granulated sugar, lemon juice, and lemon zest, if using, until smooth.
• Cook. Set the bowl over a pot of gently simmering water to create a double boiler. Cook, whisking constantly, until the curd thickens and coats the back of a spoon, about 8 to 10 minutes.
• Finish. Remove from heat and whisk in the butter cubes until smooth and silky.
• Chill. Press plastic wrap directly onto the surface of the curd and refrigerate for 1 to 2 hours, until fully thickened.

Yeast Donuts

• Warm milk and activate yeast. Warm the milk until lukewarm, about 105–110°F. Stir in 1 teaspoon of the measured sugar, then sprinkle the yeast over the top. Let sit for 5 to 10 minutes, until foamy.
• Mix wet ingredients. In a large mixing bowl, combine the beaten egg, melted butter, remaining sugar, and salt.
• Combine. Stir in the foamy yeast mixture until blended.
• Add flour. Gradually add the flour, about 1 cup at a time, mixing until a shaggy dough forms.
• Knead. Turn the dough out onto a lightly floured surface and knead for 8 to 10 minutes, until smooth, elastic, and slightly tacky. Add small amounts of flour only if needed.
• First rise. Place the dough in a greased bowl, turning once to coat. Cover and let rise in a warm place for 1 to 1 ½ hours, or until doubled in size.
• Shape donuts. Punch down the dough and roll it out on a lightly floured surface to about ½-inch thickness. Cut using a 2 ½-inch donut cutter or a round cutter with a smaller cutter for the center.
• Second rise. Transfer donuts to a floured or parchment-lined baking sheet. Cover loosely and let rise for 30 to 45 minutes, until puffy and nearly doubled.
• Heat oil. Heat vegetable oil in a deep fryer or heavy pot to 375°F, filling the pot only 2 to 3 inches deep.
• Fry. Fry donuts a few at a time, turning once as they rise to the surface, until golden brown, about 1 to 2 minutes per side. For doneness, the internal temperature should reach 185–190°F.
• Drain. Remove with a slotted spoon and drain on paper towels.
• Fill. Transfer the chilled lemon curd to a piping bag fitted with a round tip. Use a small knife to create a hole in the side of each donut, then pipe about 2 to 3 tablespoons of lemon curd into each.
• Coat and serve. Roll donuts in powdered sugar to coat. Serve and enjoy.