Put the butter in a large (at least 6 cup) microwave-safe glass bowl or measuring cup. Cover bowl and set your microwave on 50 percent power, melt the butter. (~1 to 1 1/2 minutes).
Pour butter into a blender and add lemon zest, juice, sugar, eggs, egg yolk and salt. Blend until smooth. You can also use a stick blender. Pour mixture back into the same glass bowl you melted the butter in.
Microwave mixture, on full power, in 1-minute intervals for 5 minutes. Whisk furiously between intervals, especially around the edges of the bowl. It should start to thicken. (If it looks like it’s starting to thicken before 5 minutes, stop and continue to the next step; microwaves can vary.) See post for details.
Reduce power to 70 percent and continue to microwave for another 1 to 2 minutes, stopping to whisk every 30 seconds until the curd thickens enough to coat the back of a spoon. (It will continue to thicken as it chills.) An instant-read thermometer should register 180 degrees (yet another reason to get a thermometer)
Whisk well. If you think there are any coagulated bits of egg, strain curd through a fine-mesh sieve into a clean bowl. Or if it looks smooth, you can leave it in its current bowl. Press a piece of plastic wrap against the surface to create an airtight seal and let the curd cool to room temperature. Refrigerate until cold, at least 3 hours.
Enjoy! Pair your curd with fresh berries, scones, or fold it into whipped cream for cake filling. Eat it off a teaspoon!