Separate the yolks from the whites. Zest and juice the lemons.

Use a wire whisk to whisk together the sugar and egg yolks in a medium saucepan. Stir in the lemon zest then gradually stir in the lemon juice and salt.

Place pan on low heat and stirring constantly with a wire whisk (or use a rubber spatula) until  mixture thickens (it should be thick enough to coat the back of a wooden spoon) and has reached 170 F. Turn heat up to medium and continue to whisk or stir constantly if mixture has not thickened enough after about ten minutes. 

Remove pot from heat, then add the cold, cubed butter. Stir until the butter has melted.

Pour curd into a sealable, heat safe container such as a jar with a lid or a bowl. If you've poured it into a bowl, place some plastic wrap directly on top to prevent a skin from forming. 

In a small bowl, combine the yeast, warm water and a pinch of sugar. Stir til yeast is dissolved. Let stand until foamy, about 5 minutes.**  Once foamy, add the yeast mixture to the bowl of an electric mixer fitted with the hook attachment. Add the milk, butter, egg and egg yolks, sugar, salt,cinnamon, and flour. Beat on low until a soft dough forms. Increase the speed to medium  and beat 2-3 more minutes. Stop the mixer and check the dough. It should be sticky, but if it's too sticky to handle, add 1/4 cup more flour. If it's still too sticky, add an additional 1/4 cup ( you want the dough to be somewhat sticky so be careful not to add too much flour.

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 2 hours. 

Turn dough out onto a well floured surface and roll it out with a rolling pin til dough is 1/2 inch thick. (Lightly flour both your rolling pin and donut cutter). Use a 3 inch donut cutter to cut out as many rounds as possible. Place donuts on a large lightly floured large 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 350 F. While the oil warms, line baking sheets with paper towels and take out a slotted spoon. 

Fry donuts (about 2 at a time) for around 1 minute on each side, or until golden brown. Recheck the temperature of the oil periodically as you go, so as not to let the oil get too cool or too hot. Adjust temprature of oil as needed. Drain on paper towels.  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 (not the end you eat with) into the side of a donut to create some space for the curd.

Gently squeeze lemon curd into the donut. Enjoy!