Cut the butter into four pieces. Place the butter in a medium heat proof glass mixing bowl. Place a large fine sesh sieve over the bowl. Set aside.

Add the sugar and lemon zest to a medium pot. Use your fingertips to rub the zest into the sugar.

Pour the lemon juice, pinch of salt and eggs into the sugar and whisk to combine. Try to break up the egg whites while you whisk. Set pot over low heat.

Stir constantly with a silicone whisk while the curd cooks and until the curd thickens. Cook for about 10 minutes or until the curd registers 160 to 170 F on an instant read thermometer. You can also check for doneness using the spoon method. Dip a wooden spoon into the curd and run your finger across the back of the spoon to make a line. If the line stays clear, the curd is ready.

Remove pot from heat. Pour the curd through the sieve into the butter. Use a spatula to help press the curd throught the sieve. Set sieve aside then stir curd til the butter has melted completely. Pour the curd into an airtight container. Allow to cool til room temperature before covering the container. Set in the fridge to chill til you're ready to use it.

Heat the milk in the microwave til it reaches 105 F -115 F. Stir in the yeast and 2 tablespoons of sugar til yeast is dissolved. Let stand until foamy, about 5 minutes.

In the bowl of an electric stand mixer fitted with the hook attachment, add the flour, salt, butter, eggs and 1 tablespoon granulated sugar.

Once the yeast is foamy, add it to the bowl with the rest of the ingredients. Beat on low until a soft dough forms. Increase the speed to medium and beat for 10-15 more minutes or until the dough passes the windowpane test.

Lightly grease a large mixing bowl with oil or butter and transfer dough to that bowl. Cover bowl with plastic wrap and a kitchen towel. Let dough rise in a draft free place at warm room temperature until doubled in bulk, 1 to 1½ hours.

Once it has doubled in size, punch dough down to remove excess air. Turn dough out onto a floured surface and roll it out with a rolling pin til dough is ½ inch thick.

Use a 3 inch donut cutter to cut out as many rounds as possible. Place donuts on a large lightly floured baking sheet, being careful not to place them too close together.

Cover donuts with a clean kitchen towel and let rise in a warm, draft free room at room temperature until they appear slightly puffed, about 30 minutes.

About 15 minutes before the donuts finish rising, add about 3 inches of oil to a heavy-bottomed pot or Dutch oven over medium heat and heat it to 355 F.

Fry donuts (about 3 at a time) for around 1 minute or minute and a half on each side, or until golden brown. Drain on paper towels briefly then roll the donuts in granulated sugar or granulated sugar mixed with lemon zest while donuts are still warm.

Cool to room temperature before filling with lemon curd. To fill donuts, transfer lemon curd to a pastry bag with a wide tip. Insert the end of a spoon into the side of a donut to create some space for the curd. Gently squeeze lemon curd into the donut.