In a microwave safe bowl combine your cooled melted butter with the other ingredients needed. Whisk until everything is well combined and the sugar looks mostly dissolved. No chunks.
Microwave for 2-3 minutes, stopping every 30- 40 seconds to stir. Keep a close eye towards the end. Stop cooking it as soon as you notice it "pile up" on the back of your spoon and create little peaks that don't slide right off. 15 seconds too long can make or break this and cause you to restart.
Once the curd has finished cooking, let it cool on the counter or place it in the freezer for a few minutes while you prep your mug cake.
Lemon Mug Cake
In a microwave safe mug combine your flour, salt and baking powder. Mix well and make sure there are no baking powder chunks.
Add in the sugar, milk, coconut oil, lemon juice, and lemon zest. Mix until combined and no chunks remain. If the coconut oil chunked up as a result of the cold milk, that's a different story and fine. Just note that we aren't looking for lumpy pancake batter here.
Top with 1 tablespoon of your lemon curd. There should be enough curd in the bowl to make 2 or 3 muffins.
Microwave for 1 min 30 seconds to 2 minutes. Microwaves vary and my small cheap one takes a full 2 minutes for this cake. It will still look slightly wet on top, you can use a fork to pull up cake from the bottom to ensure that it's fully cooked.
Glaze
While cake is cooking mix together the ingredients for the glaze. This makes enough for topping a single cake, so double or triple as necessary.
Pull the warm lemon cake from the microwave and top with the glaze. Let cool slightly and then enjoy.
Notes
Microwaves can vary, so closely monitor the cake and lemon curd as they cook, as even a few seconds can affect the outcome.The lemon curd's egg yolk should reach a temperature of over 160°F after about 2 minutes in the microwave, ensuring it's thoroughly cooked.Exercise caution when handling hot items from the microwave, as they will be very hot.If you have an oven-safe coffee mug, you can bake the cake in the oven as directed, placing it on a baking sheet and baking at 350°F for 10 minutes.If you don't have a lemon for juice, you can use store-bought lemon juice instead of zesting a lemon. The flavor may be slightly different but still delicious.