Zest your lemons with a zester or box grater. This works best with cold lemons (versus room temperature.)
Slice the lemons in half and juice them with a citrus juicer. Combine the zest and juice in a liquid measuring cup to equal 1/2 cup of juice. If you don't have enough, you can juice more lemons or add bottled lemon juice.
Separate six eggs - for this recipe, we will only use the yolks. Reserve the whites for another use.
Prepare a double boiler insert on the stove on heat to medium. Put the egg yolks in the double boiler insert with 3/4 cup sugar, and whisk for a minute to combine and dissolve sugar.
Next, add the lemon juice and zest and switch to a wooden spoon. Keep an eye on your burner heat, you may need to turn it down as you go. Continually stir for 15 minutes or so (it may take less or more time, depending on your heat level.)
Meanwhile, cut your stick of unsalted butter into slices and set aside.
As you stir, it will start to thicken. You will know it is done when it is thick enough to stick to the back of the wooden spoon. At this point, it will also have a pudding-like consistency and a golden yellow color. When you determine it's done, turn off the heat and stir in your sliced stick of butter until it melts.
At this point, you can run the finished curd through a fine-mesh strainer to filter out any lemon zest or potential cooked egg bits. Sometimes I do this process, sometimes I don't!
Store in an airtight container in the refrigerator for up to a few weeks, or it freezes well.
Enjoy your lemon curd with your favorite dessert or breakfast!